fix(codegen): generate default endpoint provider for default regional AWS endpoints

[kuhe]

Issue

smithy-lang/smithy-typescript#1643 must be merged and codegen updated before merging this PR.

edit: The above PR is related but not required

Description

When a client uses the default regional AWS endpoint ruleset from our code generator, it should now also receive a config resolver that adds a default provider function for client.config.endpoint().

This is because pre-endpoints-2.0 it was guaranteed that config.endpoint() would exist as a function. This was removed for public AWS SDK clients because we have no way of guaranteeing an endpoint is known without knowing the operation being called, but for internal generated clients this should be retained when using the default regional AWS endpoints, because they are a direct replacement for endpoints.json-based endpoints.

Testing

additionally, protocol test clients qualify as non-public AWS clients, and behave in the same way as described above.

// smithy client (non-AWS), endpoint is required.
const smithy = new SmithyClient({ endpoint: "" });
await smithy.config.endpoint(); // unchecked call

// AWS SDK public client, endpoint is optional, config.endpoint is not guaranteed
const aws = new AwsSdkClient({});
await aws.config.endpoint?.(); // call must be checked.

// AWS non-public client, endpoint is optional but default regional endpoints are applied
const awsInternal = new AwsInternalClient({});
await awsInternal.config.endpoint(); // unchecked call

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