Allow remote opensearch connection checks in datanode rest api proxy

[todvora]

New remote-reindex connection check in the datanodes needs to be accessible by default through our datanode proxy implementation in the graylog server. We need to add it to the default allowlist.

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]

lgtm. thank you