Add support for SSO bearer token authentication to the SDK (#3453)

This PR adds support for SSO bearer token authentication to the AWS SDK,
specifically for Code Catalyst, which requires authentication via SSO
with a Builder ID using a bearer token rather than SigV4.

This functionality was developed in a feature branch, and this PR merely
merges that branch to main. The changes consist of the following
previous PRs:
- https://github.com/smithy-lang/smithy-rs/pull/3381
- https://github.com/smithy-lang/smithy-rs/pull/3442
- https://github.com/smithy-lang/smithy-rs/pull/3443

All these changes have been reviewed in the previous PRs, but it would
be good to review this again as a whole to verify it all looks good.

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