* [vcpkg registries] support versions
This PR merges the Registries changes and the versioning changes, so that one can use both at the same time.
There is one major difference between this PR and the RFC (#13590), which is that instead of version files looking like:
```json
[
...
]
```
version files look like:
```
{
"versions": [
...
]
}
```
this is to support interop between this PR and existing demos and the like;
fixing this, along with perhaps renaming `port_versions` to `port-versions` should be done after this is merged,
should be a trivial change.
