filters: optimize cel expression context with constant-time lookups

[agrawroh]

Description

This PR optimizes the CEL expression context implementation by replacing if-else chains with hash maps for O(1) lookups and improving memory management. This would enhance the performance for header and filter state access patterns while maintaining the same functionality.

This change introduces constant-time lookups using absl::flat_hash_map and improves the overall design through static initialization of lookup maps for better performance.

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Commit Message: filters: optimize cel expression context with constant-time lookups
Additional Description: This commit replaces linear if-else chains with flat_hash_maps for property lookups in the CEL expression context. It also improves memory management and provides better encapsulation of implementation details.

Risk Level: Low

• No functional changes
• Only performance optimizations
• Existing test coverage maintained

Testing:

• All existing tests pass
• No behavioral changes

Docs Changes: N/A - internal implementation change only
Release Notes: N/A

[repokitteh-read-only]

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🐱

Caused by: #37057 was opened by agrawroh.

see: more, trace.

[wbpcode]

Thanks. This is great. Any performance improvements are welcome if it does't break the security and our style.

But could you also add a benchmark test to prove it works as our expection? Thanks.

/wait