EndpointResolver Upgrade Guidance

[rcoh]

The aws-smithy-http::endpoint module has been removed.

If you previously defined a custom endpoint resolver for a service, you will now use the trait defined directly by the service, <service>::config::endpoint::ResolveEndpoint.

• This trait does not have a generic anymore. Instead, Params is the endpoint::Params type for the given service
• The return type is now a future. Your previous code can be adapted with EndpointFuture::ready
• The error type is now Box<dyn Error + Send + Sync>