feat(server): Remote Materialization

[aniketpalu]

Summary

Supersedes #6590 (clean single-commit history with proper DCO).
Add ?async=true query parameter support to the existing /materialize and /materialize-incremental endpoints. When set, materialization runs in a background thread and the endpoint returns 202 Accepted immediately. Concurrent requests for an already-MATERIALIZING FV are rejected with 409. Also adds ?force=true to allow operators to override stuck MATERIALIZING state (e.g. after a crashed SparkApplication pod).

Client-side (SDK)

• store.materialize(..., remote=True) delegates to the feature server
  (URL/TLS derived from online_store config)
• wait=False returns immediately after triggering
• store.poll_materialization() for status polling via registry state

Server-side

• ?async=true on existing /materialize and /materialize-incremental
• ?force=true to skip the 409 concurrency guard
• Four module-level helpers extracted for testability

Registry

• SQL and Snowflake apply_materialization() now set FV state to
  AVAILABLE_ONLINE (parity with file-based registry)

Which issue(s) this PR fixes

Addresses #4526

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Misc