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
• Update to aws-crt-swift 0.41.0 with support for account ID in AWS credentials

Note: Bump minor version when this feature ships

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.