[WIP][python] Add user identity-based token cache for RESTTokenFileIO

[shyjsarah]

Purpose

This PR implements user identity-based token cache isolation in RESTTokenFileIO to prevent multiple users from sharing the same data token when accessing the same table location.

Problem: The previous implementation used only table identifier as the token cache key, causing token cache pollution when multiple users accessed the same table within a single process. All users would share the same data token regardless of their actual authentication credentials.

Solution:

• Use (path, user_identity) as the token cache key to ensure proper isolation between different users
• Extract user identity from authentication provider (Bear Token, DLF AccessKey, ECS Role, STS File, etc.) - all use actual access_key_id
• Implement double-check locking pattern for thread-safe token caching
• Path-based caching avoids issues with table renames (physical location remains constant)

Key Changes:

1. Add user_identity parameter to RESTTokenFileIO.__init__() - stores user identity for cache key construction
2. Add _extract_user_identity() method in RESTCatalog to extract user identity from auth provider:
   • Bear Token → "bear:{token}"
   • DLF AccessKey/ECS Role/STS File → "dlf:{access_key_id}" (actual AK from token, e.g., STS.xxx)
3. Modify try_to_refresh_token() to use (path, user_identity) tuple as cache key
4. Implement thread-safe token cache with double-check locking (instance lock + global cache lock)
5. Pass user_identity when creating RESTTokenFileIO instances in file_io_for_data()

Tests

New Unit Tests (test_token_cache_isolation.py):

• test_different_users_have_separate_token_cache - Verifies different users get separate tokens for same path
• test_same_user_reuses_token_cache - Verifies same user reuses cached token
• test_table_rename_preserves_token_cache - Verifies table rename doesn't break token cache (path-based)
• test_empty_user_identity_isolation - Verifies empty user identity handling
• test_token_cache_with_expiry_check - Verifies token expiration checking logic