Adding minimal support for accountId related endpoint rules built-ins (#3792)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
This change adds minimal support for the `AWS::Auth::AccountId` and
`AWS::Auth::AccountIdEndpointMode` endpoint built-ins. Endpoint rules
containing these builtins will generate correct code, but we do not
support user setting of these values (or any other features defined in
the spec for account id based endpoint resolution).

## Description
<!--- Describe your changes in detail -->
* Add option to endpoint builtins decorator to skip the `SdkConfig`
related codegen
* Add `AccountId` and `AccountIdEndpointMode` built-ins to the list of
generated ones
* Update the test input generator to add a `us-east-1` region if the
test provides built-ins but does not specify a region
* Add the documentation string from the test definition to the generated
test code
* Update tests for `EndpointBulitInsDecorator` to include the two new
bulit-ins

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
* Update tests for `EndpointBulitInsDecorator` to include the two new
bulit-ins
* Ran our CI against this change with the updated DDB endpoints rules
and all tests passed.
https://github.com/smithy-lang/smithy-rs/actions/runs/10423338690



## 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.

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Co-authored-by: ysaito1001 <awsaito@amazon.com>