Add support for parsing and merging AWS config and credentials files

[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]

My bad, clicked approve by mistake. Only the test comments are blocking

[jonathan343]

Thanks Ujjwal. Commenting on a few edge cases that seem like bugs to me. Still getting through others, but this is what I have so far.

[jonathan343]

Thanks Ujjwal, this pretty much looks good to me. Had one follow up from a previews review and a comment on the missing changelog entry. Can you add a feature entry to smithy-aws-core with something like below:

Add utilities for parsing and merging shared AWS configuration and credentials files. These utilities are not yet integrated into generated clients.

[jonathan343]

Thanks, this fixes the original example, but it still fails when the invalid property is the first property in the section:

[profile p]
invalid key = ignored
  continuation

This currently raises:

ConfigParseError: Line 3: Expected a property definition, found continuation

current_key is checked before ignore_continuation, so the continuation is only ignored when an earlier valid property populated current_key. Could we check ignore_continuation first and reset it when entering a new section? It would also be useful to test this case without a preceding valid property.