Implement core Smithy endpoint support (#183)

* Implement core Smithy endpoint support

This commit adds `Endpoint` to smithy-http & records our design decisions in `endpoint.md`. This provides support for the endpoint trait in Smithy. A design for endpoint discovery is proposed but is not currently implemented.

* Apply suggestions from code review

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>

* More cleanups

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>