sts:AssumeRole does not honour proxy settings

Credentials configured with a role do not use proxy settings on `sts:AssumeRole` call.

To fix, 7904367ac23a673be0d6c8500e420d614cd10015 needs to be ported to d12ab1def72f1577abf282a5d223aaa17f22c68e by passing `clientConfiguration` on calls to assumeRole:

https://github.com/jenkinsci/aws-credentials-plugin/blob/48985bce9bdc79a234c8e31ea51458e03bd52269/src/main/java/com/cloudbees/jenkins/plugins/awscredentials/AWSCredentialsImpl.java#L124
https://github.com/jenkinsci/aws-credentials-plugin/blob/48985bce9bdc79a234c8e31ea51458e03bd52269/src/main/java/com/cloudbees/jenkins/plugins/awscredentials/AWSCredentialsImpl.java#L140
https://github.com/jenkinsci/aws-credentials-plugin/blob/48985bce9bdc79a234c8e31ea51458e03bd52269/src/main/java/com/cloudbees/jenkins/plugins/awscredentials/AWSCredentialsImpl.java#L215

Relevant stack trace of master node behind proxy using `iamRoleArn`
```
hudson.remoting.ProxyException: java.net.SocketTimeoutException: connect timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:339)
at com.amazonaws.http.conn.ssl.SdkTLSSocketFactory.connectSocket(SdkTLSSocketFactory.java:142)
at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:142)
Caused: hudson.remoting.ProxyException: org.apache.http.conn.ConnectTimeoutException: Connect to sts.amazonaws.com:443 [sts.amazonaws.com/52.46.134.192] failed: connect timed out
at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:151)
at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:373)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.amazonaws.http.conn.ClientConnectionManagerFactory$Handler.invoke(ClientConnectionManagerFactory.java:76)
at com.amazonaws.http.conn.$Proxy81.connect(Unknown Source)
at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:381)
at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:237)
at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:185)
at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:56)
at com.amazonaws.http.apache.client.impl.SdkHttpClient.execute(SdkHttpClient.java:72)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeOneRequest(AmazonHttpClient.java:1238)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeHelper(AmazonHttpClient.java:1058)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.doExecute(AmazonHttpClient.java:743)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeWithTimer(AmazonHttpClient.java:717)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.execute(AmazonHttpClient.java:699)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.access$500(AmazonHttpClient.java:667)
at com.amazonaws.http.AmazonHttpClient$RequestExecutionBuilderImpl.execute(AmazonHttpClient.java:649)
at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:513)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.doInvoke(AWSSecurityTokenServiceClient.java:1307)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.invoke(AWSSecurityTokenServiceClient.java:1283)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.executeAssumeRole(AWSSecurityTokenServiceClient.java:466)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.assumeRole(AWSSecurityTokenServiceClient.java:442)
at com.cloudbees.jenkins.plugins.awscredentials.AWSCredentialsImpl.getCredentials(AWSCredentialsImpl.java:124)
at com.cloudbees.jenkins.plugins.amazonecr.AmazonECSRegistryCredential.getPassword(AmazonECSRegistryCredential.java:128)
at com.cloudbees.jenkins.plugins.amazonecr.AmazonECSRegistryTokenSource.convert(AmazonECSRegistryTokenSource.java:55)
at com.cloudbees.jenkins.plugins.amazonecr.AmazonECSRegistryTokenSource.convert(AmazonECSRegistryTokenSource.java:41)
at jenkins.authentication.tokens.api.AuthenticationTokens.convert(AuthenticationTokens.java:148)
at jenkins.authentication.tokens.api.AuthenticationTokens.convert(AuthenticationTokens.java:110)
at org.jenkinsci.plugins.docker.commons.credentials.DockerRegistryEndpoint.getToken(DockerRegistryEndpoint.java:185)
at org.jenkinsci.plugins.docker.commons.credentials.DockerRegistryEndpoint.newKeyMaterialFactory(DockerRegistryEndpoint.java:243)
at org.jenkinsci.plugins.docker.workflow.RegistryEndpointStep$Execution.newKeyMaterialFactory(RegistryEndpointStep.java:88)
at org.jenkinsci.plugins.docker.workflow.AbstractEndpointStepExecution.start(AbstractEndpointStepExecution.java:44)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:229)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:153)
at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:122)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:48)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.methodCall(DefaultInvoker.java:20)
Caused: hudson.remoting.ProxyException: com.amazonaws.SdkClientException: Unable to execute HTTP request: Connect to sts.amazonaws.com:443 [sts.amazonaws.com/52.46.134.192] failed: connect timed out
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.handleRetryableException(AmazonHttpClient.java:1116)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeHelper(AmazonHttpClient.java:1066)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.doExecute(AmazonHttpClient.java:743)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeWithTimer(AmazonHttpClient.java:717)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.execute(AmazonHttpClient.java:699)
at com.amazonaws.http.AmazonHttpClient$RequestExecutor.access$500(AmazonHttpClient.java:667)
at com.amazonaws.http.AmazonHttpClient$RequestExecutionBuilderImpl.execute(AmazonHttpClient.java:649)
at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:513)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.doInvoke(AWSSecurityTokenServiceClient.java:1307)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.invoke(AWSSecurityTokenServiceClient.java:1283)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.executeAssumeRole(AWSSecurityTokenServiceClient.java:466)
at com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient.assumeRole(AWSSecurityTokenServiceClient.java:442)
at com.cloudbees.jenkins.plugins.awscredentials.AWSCredentialsImpl.getCredentials(AWSCredentialsImpl.java:124)
```