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.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Key design points:

• AsyncAwsConfig cannot be instantiated directly. Calling AsyncAwsConfig() raises a ConfigError. All construction must go through await AsyncAwsConfig.resolve(), which ensures every field is resolved and validated before the object is returned.
• All config fields are resolved upfront in one call. There is no lazy resolution.
• Resolution follows a strict precedence: explicit overrides > environment variables > config file profile > default value.
• Every field tracks where its value came from (provenance), queryable via config.source_of("field_name").

Usage

Direct instantiation is blocked:

from smithy_aws_core.config import AsyncAwsConfig

config = AsyncAwsConfig()
# raises ConfigError: "AsyncAwsConfig cannot be constructed directly.
#   Use `await AsyncAwsConfig.resolve(...)` instead."

Resolve from environment variables:

If AWS_REGION=us-west-2 is set in the environment:

config = await AsyncAwsConfig.resolve()
print(config.region)  # "us-west-2"

Resolve from config file:

If ~/.aws/config contains:

[profile default]
region = us-east-1
retry_mode = standard
max_attempts = 10

And no environment variables override these:

config = await AsyncAwsConfig.resolve()
print(config.region)        # "us-east-1"
print(config.retry_mode)    # "standard"
print(config.max_attempts)  # 10

Override specific values:

Overrides passed to resolve() take highest precedence — the value is validated but skips the env/file resolution chain entirely:

config = await AsyncAwsConfig.resolve(region="eu-west-1", max_attempts=5)
print(config.region)  # "eu-west-1" (not from env or file)

Use a named profile:

config = await AsyncAwsConfig.resolve(profile="some_name")

If the profile doesn't exist:

config = await AsyncAwsConfig.resolve(profile="nonexistent")
# raises ProfileNotFoundError: "Profile 'nonexistent' (from the profile argument)
#   not found in config file."

Provenance tracking:

config = await AsyncAwsConfig.resolve()
config.source_of("region")        # ConfigSource.ENV
config.source_of("retry_mode")    # ConfigSource.DEFAULT
config.source_of("max_attempts")  # ConfigSource.PROFILE

Override values after resolution:

Values can be changed after resolution using plugins. The source updates to OVERRIDE and the new value is validated:

config = await AsyncAwsConfig.resolve()
config.region = "ap-southeast-1"
config.source_of("region")  # ConfigSource.OVERRIDE

config.region = "some-value!"  # raises ConfigValidationError

[jonathan343]

LGTM, thanks Ujjwal.

There are some UX things we'll want to clean up. But this is a good start and we have other work that depends on this, so I'm approving.