Add initial RFC document (endpoints 2.0) (#1637)

* Add initial RFC document

* CR feedback

* RFC updates

* Rename doc, add to summary / overview

* Apply suggestions from code review

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Update design/src/rfcs/rfc_0020_endpoints_20.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* RFC updates

* More cleanups

* Delete old file

* rfc updates

* delete old file

* fix merge

Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>