Add support for hosting 'utilities' and 'files' plugins on the Hub

As discussed in meltano#2605 (comment 852917503).

The discovery.yml paradigm requires stability that is not available for bleeding-edge releases and more volatile or "beta stage" plugin integrations. The way we mitigate this presently is to give the users instructions on how to do a --custom install, but having these in the Hub would give a nice middle ground. Furthermore, as the "lock" behavior becomes default in meltano add, we will be free to make what today be "breaking changes" here in the hub, but without actually breaking any users (since definitions will be frozen).

1. Let's add support for hosting 'utilities' and 'files' plugins here in the Hub. (This issue.)
2. Let's build the mechanism for meltano add to install references directly from the Hub. (Support `meltano add` directly from Hub (meltano#3283 - closed))