Add async resolver for AWS config values

[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 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, FileSystem, validator enforcement, and construction blocking, and verified that they all pass.
• All existing tests pass

Note: This is a revision to the old PR (#738) that was automatically closed after the feature branch was merged.

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[jonathan343]

We are promising to return int | None for max_attempts, however, this isn't true when a customer passes in a str. We cannot violate that contract.

>>> config = await AsyncAwsConfig.resolve(max_attempts="5")
>>> type(config.max_attempts)
<class 'str'>

[jonathan343]

When a customer provides a profile value that doesn't actually exist, we throw a misleading error:

>>> c>>> config = await AsyncAwsConfig.resolve(profile="FOOBAR")  from smithy_aws_core.config import AsyncAwsConfig
>>> config = await AsyncAwsConfig.resolve(profile="FOOBAR")
Traceback (most recent call last):
  File "/Users/gytndd/.local/share/uv/python/cpython-3.12.13-macos-aarch64-none/lib/python3.12/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/gytndd/.local/share/uv/python/cpython-3.12.13-macos-aarch64-none/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "<console>", line 1, in <module>
  File "/Users/gytndd/dev/GitHub/temp6/smithy-python/packages/smithy-aws-core/src/smithy_aws_core/config/aws_config.py", line 105, in resolve
    await instance._resolve_fields(overrides)
  File "/Users/gytndd/dev/GitHub/temp6/smithy-python/packages/smithy-aws-core/src/smithy_aws_core/config/aws_config.py", line 165, in _resolve_fields
    spec.validator(getattr(self, field_name))
  File "/Users/gytndd/dev/GitHub/temp6/smithy-python/packages/smithy-aws-core/src/smithy_aws_core/config/validators.py", line 24, in validate_region
    raise ConfigValidationError(
smithy_aws_core.config.exceptions.ConfigValidationError: Invalid value for 'region': None. Region is required and must be set.

A common case will be when customers make a type in the profile name. We should provide a more accurate/helpful error.

[jonathan343]

Usually DeprecationWarning means something works for this package today and won't in the future. We should use UserWarning instead since this is by design.