Make coordination Ra system parameters configurable via rabbitmq.conf

[kjnilsson]

Expose all coordination Ra system parameters through new coordination.* configuration options, following the pattern established in PR #15962 for quorum queues.

This allows operators to fine-tune Raft log and WAL behavior specifically for the coordination system without affecting other Ra-based systems (quorum queues, Khepri).

New coordination.* configuration options:

• coordination.wal_compute_checksums (default: true)
• coordination.segment_compute_checksums (default: true)
• coordination.wal_max_size_bytes (default: 64 MB)
• coordination.wal_max_entries (default: from Ra's default_config)
• coordination.wal_max_batch_size (default: from Ra's default_config)
• coordination.segment_max_size_bytes (default: from Ra's default_config)
• coordination.segment_max_entries (default: from Ra's default_config)
• coordination.max_append_entries_rpc_batch_size (default: from Ra's default_config)
• coordination.compress_mem_tables (default: true)
• coordination.snapshot_chunk_size (default: from Ra's default_config)

Each coordination.* setting has a fallback to its default value, ensuring backward compatibility. The wal_max_size_bytes parameter maintains the existing hardcoded default of 64 MB.

Implementation details:

1. Added macros for coordination defaults in rabbit_ra_systems.erl
2. Refactored get_config(coordination) to read from application environment variables with sensible defaults, matching the quorum_queues pattern
3. Added Cuttlefish schema mappings for all new configuration options
4. Added comprehensive documentation in rabbitmq.conf.example
5. Added schema validation tests in rabbit.snippets
6. Added dedicated test suite (rabbit_ra_systems_SUITE) to validate defaults

All configuration is explicitly passed to the coordination Ra system during initialization, making the data flow transparent and maintainable.

Backward compatibility: Existing deployments continue to work unchanged, with coordination.wal_max_size_bytes defaulting to 64 MB.