Add support for config resolver

[ubaskota]

Issue #, if available:

Description of changes:
This change implements the async config resolution pipeline with AsyncAwsConfig using a resolve() classmethod as the only construction path. It adds SharedConfigContext for holding env vars and memoized config file data, FieldSpec for per-field resolution metadata, and provenance tracking via ConfigSource. Resolvers for region and retry_strategy_options are included along with validators for both fields.

Testing:

• Added unit and end-to-end tests covering env var resolution, profile resolution, explicit overrides, defaults, provenance tracking, validator enforcement, and construction blocking, and verified that they all pass.
• All existing tests pass

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