[google-auth-library-java] How to set a timeout for GoogleCredentials?

`GoogleCredentials` can be initialized with an optional `HttpTransportFactory`. But this doesn't provide any means of setting the connection and read timeout for the HTTP requests made by the credentials implementation. Is there a way to set these timeout values for `GoogleCredentials`?

More context: In the Google HTTP client, the recommended way to set timeouts is via an `HttpRequestInitializer` that gets passed into the `HttpRequestFactory`.

```
myTransport.createRequestFactory(new HttpRequestInitializer() {
@Override
public void initialize(HttpRequest request) throws IOException {
request.setConnectTimeout(10000);
request.setReadTimeout(10000);
}
});
```

But currently there appears to be no way to specify a request initializer for `GoogleCredentials`. As a result, there's no simple way to configure the timeouts for the HTTP requests made by this implementation.

This is a somewhat critical issue, since `GoogleCredentials` get invoked before any other HTTP requests are made by the client. The default timeout settings of `GoogleCredentials` essentially mask and override any timeout values set in the client.

```
myTransport.createRequestFactory(new HttpRequestInitializer() {
@Override
public void initialize(HttpRequest request) throws IOException {
credentials.initialize(request);
request.setConnectTimeout(10000);
request.setReadTimeout(10000);
}
});
```

In the above example you expect requests to timeout after 10 seconds. But because credentials are in the initializer chain, it won't timeout until 60s (the default).

从 GoogleCredentials 及其 HttpTransportFactory 路径开始，然后检查 HttpRequestInitializer 如何应用于 HttpRequestFactory。只有当调用方能够为凭据 HTTP 请求配置连接超时和读取超时，且这些设置不会被凭据初始化器覆盖时，此更改才算完成。

由索引模型根据 Issue 内容生成。