Identify/load oAuth2 services dynamically

As Eloy mentioned in MDL-70689: I'd suggest, of course in another issue, to go one step forward towards services isolation, being able to make all different services really dynamic, so there isn't any dependency in core about any of them.

Imagine, to tell it with other words, that we want to make them "subplugins" (not saying they should, just the idea of tool_oauth, not knowing about them in advance).

Or, maybe easier to understand, issuers.php shouldn't have any "google" or "facebook" occurrence. Somehow... all the available types must be detected and presented.