This reduces bootstrap cost for Windows customers, resolving the issue initially submitted as #12502 .

The `toolsrc` tree was extracted to https://github.com/microsoft/vcpkg-tool. `bootstrap.sh` was changed to download the right source tarball, extract, and build it. This was chosen over the previous attempt, a submodule, over concerns of accidentally destroying people's local modifications.

* [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.