Cluster adapter provides count of manager-eligible nodes

[todvora]

/nocl this is just infrastructure for https://github.com/Graylog2/graylog-plugin-enterprise/pull/15041

Description

Implement access to manager-eligible nodes count in all three storage modules.

Motivation and Context

Needed for https://github.com/Graylog2/graylog-plugin-enterprise/pull/15041

How Has This Been Tested?

Added unit test

Screenshots (if appropriate):

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 requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.