Integrate identity chain into SDK client construction

[arandito]

Description

This PR integrates the new IdentityChain into generated AWS clients. A new AwsIdentityIntegration builds the default chain into aws_credentials_identity_resolver at client setup.

The integration also pins the identity resolver in the config's deep-copy memo ({id(resolver): resolver}) so it's shared by reference. Every operation deep-copies the config to isolate per-call plugin mutations, which would otherwise duplicate the resolver on each call; pinning it keeps the chain built once per client and reused across every operation.

Important

This branch builds on top of #758. Only the latest commit is the change under review here. Every earlier commit belongs to #758.

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[Alan4506]

Flagging from #773 (comment):

Here the generated setup passes config_context.profile_name into IdentityChain.create(), but the context's profile_name is never None: it falls back to AWS_PROFILE, then "default". See

smithy-python/packages/smithy-aws-core/src/smithy_aws_core/config/context.py

Lines 184 to 201 in c0150aa

	def _resolve_profile_name(
	self, explicit_profile: str | None
	) -> tuple[str, str | None]:
	"""Determine the active profile name and where it came from.
	Priority: explicit argument > AWS_PROFILE env var > "default"
	:returns: Tuple of (profile_name, origin), where origin describes the
	source for error messages and is None when the name was defaulted.
	"""
	if explicit_profile is not None:
	return explicit_profile, "the profile argument"
	env_profile = os.environ.get(_PROFILE_ENV_VAR)
	if env_profile is not None:
	return env_profile, _PROFILE_ENV_VAR
	return _DEFAULT_PROFILE, None

But #773's environment provider defers whenever profile_name is not None, so once both PRs land, a user with only env credentials and no profile configured would still get the environment provider skipped.

We should update this PR to pass the profile only when it comes from the explicit argument.

[arandito]

Added in c5498f2. Needed to improve how we track the profile source to avoid weak string comparisons. They now use ConfigSource.

Chose not to make profile a config field since it's not a request lifecycle setting. The profile name mainly influences config resolution so the resolution context is the proper layer for it.

[alexgromero]

LGTM! 🚀