feat(aws): resolve endpoints by service

[aryasaatvik]

Endpoint overrides the endpoint for every operation in context — the right shape for a full local gateway (AWS_ENDPOINT_URL, LocalStack convention). But the AWS SDKs also specify service-specific endpoints — AWS_ENDPOINT_URL_<SERVICE> and per-service services sections in ~/.aws/config — for routing only some services elsewhere: an emulator that implements a subset, a private interface endpoint for s3 only. Distilled can't express that: the operation's service is only known at request time, inside the protocol layer.

Add ServiceEndpoint, a resolver keyed by the SDK service ID exactly as it appears in the model's aws.api#service trait:

const endpoints: Record<string, string> = {
  "S3": "http://127.0.0.1:9000",
  "DynamoDB": "http://127.0.0.1:8000",
};

Effect.provideService(ServiceEndpoint, {
  resolve: (service) => endpoints[service],
})

protocol.ts and presign.ts now resolve through Endpoint.resolve(service):

1. an operation-scoped Endpoint wins,
2. then the ServiceEndpoint resolver,
3. undefined falls through to the generated Smithy endpoint rules, selecting the AWS endpoint as before.

An absent ServiceEndpoint behaves exactly as today.

The key is the SDK service ID rather than the SigV4 signing name because signing names collide — SES and SESv2 both sign as ses — while SDK IDs keep every service independently routable, and they are the identifier AWS's own service-specific endpoint configuration is derived from. Two natural follow-ups: a generated ServiceName union (from the models' sdkIds) to tighten the string key, and a ServiceEndpoint.fromEnv layer honoring AWS_ENDPOINT_URL_<SERVICE>/AWS_IGNORE_CONFIGURED_ENDPOINT_URLS.