Align auth implementation with SRA (#4198)

## Motivation and Context
This PR makes the auth implementation aligned with our design
specifications. The code changes in the PR consolidate the previous
sub-PRs that have been reviewed already:
- https://github.com/smithy-lang/smithy-rs/pull/4161
- https://github.com/smithy-lang/smithy-rs/pull/4175
- https://github.com/smithy-lang/smithy-rs/pull/4182
- https://github.com/smithy-lang/smithy-rs/pull/4183



For auth-related types users directly interact with in service config
such as `AuthScheme`, `AuthSchemeOption`, we can re-export them in a
client SDK upon a customer request.

## Testing
- All tests added in the sub-PRs above

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

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

---------

Co-authored-by: AWS SDK Rust Bot <97246200+aws-sdk-rust-ci@users.noreply.github.com>