Add ConfigResolver with shared singleton pattern

[ubaskota]

Issue #, if available:

Description of changes:

Implements ConfigResolver to resolve config values from multiple sources in precedence order. Adds get_shared_resolver() using @lru_cache to share a single resolver instance across all config objects, avoiding redundant reads from the sources.

Tests
Added unit tests to validate the expected behavior. Verified that the tests pass.

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[jonathan343]

Thanks Ujjwal.

I left some minor comments and suggestions. However, my biggest concern with this PR is the approach for config resolver sharing.

1. It doesn't follow any of the approaches discussed in previous designs
2. In cases where multiple threads hit the function before the first result is cached, it will result in each building their own resolver which is unnecessary/duplicate work