[Draft] Allow alternative trace storage engines

[scottfines]

IMPORTANT NOTE: Please do not merge this! It is part of ongoing development. Feel free to look around and make comments as you feel important, but recognize that it is a work in progress.

What is the issue

This is part of a POC for cndb-11350. The issue is that we lack the ability to connect tenants to traced queries.

What does this PR fix and why was it fixed

1. Adds a context entry to TraceKeyspace. This is almost certainly temporary (since the upgrade path for changing a system table in production seems fraught), but it allows us to proceed on other fronts in developing the POC
2. Refactors the trace storage logic to allow for alternative trace storage implementations. This way, we can write our tenant-aware trace logic on the CNDB side without needing any kind of tenancy in CC.

[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