Use hpack executable from PATH if it is newer than what stack ships with?

The latest version of `hpack` has a `--numeric-version` option. With this it would be possible for `stack` to check if there is a newer version of `hpack` installed on a users system and if so use that instead of the bundled `hpack`.

- Check if `hpack` is on the users `PATH`, if no use bundled `hpack`
- If yes, try `hpack --numeric-version`, if it fails use bundled `hpack`
- If it succeeds then check if it is newer than the bundled `hpack` and use it if so, otherwise use bundled `hpack`

This is just a suggestion, not a request to add.

(cc @scott-fleischman who did related stuff before)