Implement remote connection checks for remote-reindex migrations

[todvora]

The existing implementation has done the connection check and indices list collection from the graylog-server side. But this doesn't verify that datanodes can speak to the remote opensearch. There may be networking/routing issues, different configuration of truststores and many other problems. The real solution is to let all datanodes run the connection check and aggregate their results in the graylog server. If this check succeeds, the remote reindexing should as well.

Motivation and Context

Errors like:

PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Even with trust uknown certificates feature from #19775

How Has This Been Tested?

Manually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[moesterheld]

There is an issue when using a custom CA for OS/ES during remote reindexing. I could consistently reproduce that the connection check would give a "No indices found" error, which is gone after retrying.
Seems like a possible race condition. If we want to include it like this in the alpha build, I will approve it.