fix: Add JSON module support and correct test metadata

This commit fixes stats validation errors:

1. Add JSON module commands to commands.json:
   - JSON.GET: Return value at path in JSON serialized form
   - JSON.SET: Sets or updates JSON value at a path

2. Add 'json' group to groups.json with JSON commands

3. Fix test-suite metadata issues:
   - memtier_benchmark-playbook-rate-limiting-lua-100k-sessions.yml:
     Remove 'bitmap' from tested-groups (only scripting is tested via EVAL)

   - memtier_benchmark-playbook-realtime-analytics-membership.yml:
     Add 'sunion' to tested-commands (SUNION command was used but not listed)

   - memtier_benchmark-playbook-realtime-analytics-membership-pipeline-10.yml:
     Add 'sunion' to tested-commands

These changes resolve all stats validation errors in CI.

Author: slice4e