Add support for account-based endpoints (#4097)

## Motivation and Context
https://github.com/smithy-lang/smithy-rs/issues/3776

## Description
This PR adds support for [account-based
endpoints](https://docs.aws.amazon.com/sdkref/latest/guide/feature-account-endpoints.html)
in AWS SDKs. The code changes in the PR consolidate the previous sub-PRs
that have been reviewed already:
- #4047
- #4057
- #4087 
- #4091 
- #4094 

## Testing
- All tests added in the sub-PRs above
- Added account ID-related integration tests for DynamoDB
([commit](https://github.com/smithy-lang/smithy-rs/commit/60edd120916c1a87547951d292eae331778525ae))

## Checklist
- [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.

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