Add config file support for config resolution

[ubaskota]

Issue #, if available:

Description of changes:
Add support for parsing and merging AWS config and credentials files (~/.aws/config and ~/.aws/credentials) as part of the config resolution pipeline.

Changes:

• Custom line-by-line parser that handles inline comments, sub-properties, and property continuations
• Standardizer that normalizes section prefixes, handles [default] vs [profile default] precedence, and separates
  profiles/sso-sessions/services
• ParsedConfigFile class for querying the merged result
• Async config loading with path resolution from environment variables and defaults
• Added support to run conformance tests (config-file-parser-tests.json and config-file-location-tests.json)+ unit tests for the query layer and file I/O edge cases

Testing:

• Verified that the newly added and existing tests pass
• Manually tested against a real ~/.aws/config file to verify end-to-end parsing behavior

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[SamRemis]

[Trying to reason about this, genuine question]

This method strikes me as a little odd. If someone passes both the credentials and config path, we don't take them literally and just expand the user for them.

I don't know if this needs to be a public method. We could have all of this logic inline in load_config, or make it private, no?