Feat / add unified datasource available members endpoint

[richie912]

Proposed changes

• Added new endpoint datasources/{datasourceId}/available?search= for fetching available group members for a datasource
• Response format is consistent with the applications/{appId}/groups-members/available endpoint
• This allows replacing previous logic that required multiple endpoints

Types of changes

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

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• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules

Further comments

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