CNDB-15895: Support configurable thread usage for vector index building during compaction

[lesnik2u]

What is the issue

https://github.com/riptano/cndb/issues/15895

What does this PR fix and why was it fixed

This PR introduces the ability to configure the number of threads used for vector index building during compaction via the cassandra.sai.vector_index_build_threads system property.

By default, vector indexing attempts to utilize all available processors. This is optimal for dedicated compactors but causes excessive resource consumption on shared compactors. This change allows for restricting thread usage (e.g., setting it to 1) to ensure vector indexing resource usage is consistent with non-vector compaction tasks.

Single thread:

Semaphore limit set to 2 threads:

Max threads:

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one