Add support of Patreon auth

[umputun]

With go-pkgz/auth#95 in place, we can add Patreon as auth provider. This provider also sets a new claim allowing us to check if the user paid a subscriber and based on this info, we can do interesting things. Probably we could add optional flags, like subscribers-only.

Another thing we may consider is highlight-subscribers. This one is mostlyty for UI to make such users visually different, for example, adding some colorful frame around the avatar. We need to extend the user's info and add this flag to the config response to make it happen.

[romanilchyshyn]

From Patreon documentation:

Patreons Implementation of OAuth is not intended to serve as an authentication layer for third party services, but rather as a way to “Connect” one's Patreon user details to a preexisting account on third party services.

So, is this an intention to implement so-called connect functionality? A few questions I came with:

• Can we have authentication/authorization with a few providers simultaneously?
• How to identify such store.User?

Or it should be just authentication like for example with twitter provider?

P.S.: I'm a newbie in engine's code base & business logic so can misunderstand everything.

[umputun]

I'm not really sure what this "not intended to serve as an authentication layer" actually means. This is oauth2 and the functionality implemented by go-pkgz/auth#95 can be called "connect" or "auth" or whatever they like but in fact, this is just oauth2. We can allow patreon's users to get in and we have the info if they donating, i.e. if they subscribers or not. All we have here is the typical provider, like github or google with an extra claim is_paid_sub

[romanilchyshyn]

Let's keep it open until frontend part from comment is implemented.