Allow custom EndpointResolver to fallback

[aajtodd]

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Describe the Feature

Allow custom endpoint resolvers to fallback to the default resolver provided with the SDK. Currently when the endpoint resolver is overridden there is no way to fallback and the custom resolver has to provide endpoints for every region/service. This makes it harder for a custom resolver to override endpoints for specific regions only.

The Go v2 SDK provides a way to fallback, e.g.:

An endpoint resolver can use the EndpointNotFoundError sentinel error value to trigger fallback resolution to the service clients default resolution logic. This allows you to selectively override one or more endpoints seamlessly without having to handle fallback logic.

Is your Feature Request related to a problem?

Proposed Solution

One possible solution is to provide a sentinel endpoint, e.g.:

endpointResolver = AwsEndpointResolver { _, region -> 
     if (region == "us-west-1") myCustomEndpoint else AwsEndpoint.UseDefaultReslolver
}

Describe alternatives you've considered

Additional Context