[vcpkg registries] Registries: Take 2 (microsoft#13590)

* [vcpkg registries] Registries: Take 2

unfortunately without all the history because I really screwed up the git commands

This RFC is the new state of the world with regard to our plan for registries; it is very different from the old RFC. It includes registry databases, which are how one describes the set of versions of ports that one has access to. It also includes the specification for git registries, and updates the specification for filesystem registries with these new registry databases.

Since this RFC completely changes how registries work, the existing implementation will be broken.

* update the registries RFC for modern design

* remove bikeshedding