feat: Account ID-based endpoints

[jbelkins]

Description of changes

• The AWS-specific EndpointResolverMiddleware is renamed to AWSEndpointResolverMiddleware to make clear that it is an AWS-specialized type
• AWSEndpointResolverMiddleware is protected with Swift @_spi() to conceal it from public usage.
• Endpoint params are now created by a block which can be passed into the middleware for inline evaluation, since account ID may not be known yet when endpoint params are currently constructed.
• accountId is no longer exposed as a client config param since it is set only via AWS credential identity resolvers

Note: Bump minor version when this feature ships

Also note: This is a breaking change to the DynamoDB client because client config fields have been either removed or changed in type. Expected breakage on the customer's side is zero because those fields are currently exposed but non-functional. Only DynamoDB is affected, no other client has these config fields at this time.

New/existing dependencies impact assessment, if applicable

No new dependencies were added to this change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sichanyoo]

Initial review w/ comments about re-using extension points and removing business feature metric for ACCOUNT_ID_ENDPOINT