[Webfinger] Announce API endpoints to clients

## Is your feature request related to a problem? Please describe.

ownCloud clients have various API endpoints hardcoded, and they're mostly from old PHP world:

- `ocs/v2.php/cloud/capabilities`
- `ocs/v2.php/cloud/user`
- `remote.php/dav/meta`

In the future, we might want to get rid of the PHP, and clients would need to know about it.

## Describe the solution you'd like

`.well-known/webfinger` could announce the available API endpoints to clients

- for each endpoint could be a version announced
- each endpoint could have information about the expected format…
- authenticated `.well-known/webfinger` could announce additional endpoints, unauthenticated only the basic ones

## Describe alternatives you've considered

Don't change, and hardcode future ones (SSE) in all clients or keep adding them to the `capabilities`…

## Additional context

iOS app already can configure this via branding or MDM parameter:
- https://doc.owncloud.com/ios-app/next/appendices/mdm.html#server-endpoints

@micbar @tbsbdr

/cc @TheOneRing @felix-schwarz @JuancaG05 @jesmrec FYI