Set credentials provider for fetchMergePush

[MoeRT09]

When using Git over HTTP(s) for the remote Git repo, there was no credential provider for the fetchMergePush function. Thus those operations would fail for repositories requiring authentication. See the log output:

broker-1  | org.eclipse.jgit.api.errors.TransportException: https://xxx: Authentication is required but no CredentialsProvider has been registered
broker-1  |     at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:249) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at io.meshcloud.dockerosb.persistence.GitHandlerService.fetchMergePush(GitHandlerService.kt:125) ~[classes!/:na]
broker-1  |     at io.meshcloud.dockerosb.persistence.GitHandlerService.synchronizeWithRemoteRepository(GitHandlerService.kt:113) ~[classes!/:na]
broker-1  |     at io.meshcloud.dockerosb.persistence.GitOperationContext.synchronizeWithRemoteRepository(GitOperationContext.kt:39) ~[classes!/:na]
broker-1  |     at io.meshcloud.dockerosb.persistence.ScheduledPushHandler.pushTask(ScheduledPushHandler.kt:38) ~[classes!/:na]
broker-1  |     at jdk.internal.reflect.GeneratedMethodAccessor20.invoke(Unknown Source) ~[na:na]
broker-1  |     at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.lang.reflect.Method.invoke(Unknown Source) ~[na:na]
broker-1  |     at org.springframework.scheduling.support.ScheduledMethodRunnable.run(ScheduledMethodRunnable.java:84) ~[spring-context-5.3.30.jar!/:5.3.30]
broker-1  |     at org.springframework.scheduling.support.DelegatingErrorHandlingRunnable.run(DelegatingErrorHandlingRunnable.java:54) ~[spring-context-5.3.30.jar!/:5.3.30]
broker-1  |     at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.util.concurrent.FutureTask.runAndReset(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) ~[na:na]
broker-1  |     at java.base/java.lang.Thread.run(Unknown Source) ~[na:na]
broker-1  | Caused by: org.eclipse.jgit.errors.TransportException: https://xxx: Authentication is required but no CredentialsProvider has been registered
broker-1  |     at org.eclipse.jgit.transport.TransportHttp.connect(TransportHttp.java:687) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at org.eclipse.jgit.transport.TransportHttp.openFetch(TransportHttp.java:465) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at org.eclipse.jgit.transport.FetchProcess.executeImp(FetchProcess.java:153) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at org.eclipse.jgit.transport.FetchProcess.execute(FetchProcess.java:105) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at org.eclipse.jgit.transport.Transport.fetch(Transport.java:1480) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:238) ~[org.eclipse.jgit-6.10.0.202406032230-r.jar!/:6.10.0.202406032230-r]
broker-1  |     ... 15 common frames omitted

This PR fixes this.

[JohannesRudolph]

discuss: for my taste we started copy/pasting this around a a little too much by now and i think it makes sense to move this into a common method called by all git operations (so we don't miss HTTPS basic auth support on other git operations by accident)