Adding Sigma Repos dont use the proxy configuration

Adding Sigma repos like SigmaHQ or other repos the proxy configuration is not used like in Graylog v5.0.

## Expected Behavior

Within Graylog v5.0 we are able to get a connection with the configured proxy to the SigmaHQ and other repos.
Since we did the update to Graylog v5.1.4 we get an error and we dont see any logs for the connections to SigmaHQ etc. at our proxy.
We still the Graylog connections from other parts of Graylog at our proxy. Seems there is a issue with proxy implementation at the GitRepositoryService.

## Current Behavior

`
2023-08-25T09:18:33.170+02:00 ERROR [GitRepositoryService] Error cloning Git repository at 'https://github.com/SigmaHQ/sigma.git': https://github.com/SigmaHQ/sigma.git: connection failed
org.eclipse.jgit.api.errors.TransportException: https://github.com/SigmaHQ/sigma.git: connection failed
at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:244) ~[?:?]
at org.eclipse.jgit.api.CloneCommand.fetch(CloneCommand.java:325) ~[?:?]
at org.eclipse.jgit.api.CloneCommand.call(CloneCommand.java:191) ~[?:?]
at org.graylog.plugins.securityapp.sigma.git.GitRepositoryService.cloneRepository(GitRepositoryService.java:104) ~[?:?]
at org.graylog.plugins.securityapp.sigma.git.GitRepositoryService.cloneDefaultRepository(GitRepositoryService.java:306) ~[?:?]
at org.graylog.plugins.securityapp.sigma.rest.GitRepositoryResource.createDefault(GitRepositoryResource.java:96) ~[?:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?]
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?]
at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory.lambda$static$0(ResourceMethodInvocationHandlerFactory.java:52) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:134) [graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:177) [graylog.jar:?]
at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:219) [graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:81) [graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:478) [graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:400) [graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:81) [graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:255) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:248) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:244) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:292) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:274) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:244) [graylog.jar:?]
at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:265) [graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:234) [graylog.jar:?]
at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:684) [graylog.jar:?]
at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.service(GrizzlyHttpContainer.java:356) [graylog.jar:?]
at org.glassfish.grizzly.http.server.HttpHandler$1.run(HttpHandler.java:200) [graylog.jar:?]
at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:180) [graylog.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: org.eclipse.jgit.errors.TransportException: https://github.com/SigmaHQ/sigma.git: connection failed
at org.eclipse.jgit.transport.TransportHttp.connect(TransportHttp.java:729) ~[?:?]
at org.eclipse.jgit.transport.TransportHttp.openFetch(TransportHttp.java:465) ~[?:?]
at org.eclipse.jgit.transport.FetchProcess.executeImp(FetchProcess.java:151) ~[?:?]
at org.eclipse.jgit.transport.FetchProcess.execute(FetchProcess.java:103) ~[?:?]
at org.eclipse.jgit.transport.Transport.fetch(Transport.java:1405) ~[?:?]
at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:233) ~[?:?]
... 32 more
Caused by: java.net.ConnectException: Connection refused github.com
at org.eclipse.jgit.util.HttpSupport.response(HttpSupport.java:240) ~[?:?]
at org.eclipse.jgit.transport.TransportHttp.connect(TransportHttp.java:654) ~[?:?]
at org.eclipse.jgit.transport.TransportHttp.openFetch(TransportHttp.java:465) ~[?:?]
at org.eclipse.jgit.transport.FetchProcess.executeImp(FetchProcess.java:151) ~[?:?]
at org.eclipse.jgit.transport.FetchProcess.execute(FetchProcess.java:103) ~[?:?]
at org.eclipse.jgit.transport.Transport.fetch(Transport.java:1405) ~[?:?]
at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:233) ~[?:?]
... 32 more
`

## Possible Solution

Use the proxy configuration from server.conf for GitRepositoryService

## Steps to Reproduce (for bugs)

1. Update to Graylog 5.1.4
2. Use a proxy
3. Try to add SigmaHQ or another GIT for Sigma rules

## Context

## Your Environment

* Graylog Version: 5.1.4
* Java Version: 17
* OpenSearch Version: 5.3
* MongoDB Version: 6.0
* Operating System: SLES 15
* Browser version: any