Provide a public API for alternative logins

[ChristophWurst]

Is your feature request related to a problem? Please describe.
There are many apps that provide "alternative logins". A quick github query reveals that they all use a fragile and private API for that: https://github.com/search?q=registerLogIn+oc_app&type=Code

Describe the solution you'd like
I would like to propose a switch to a public API. There should be a service where apps can register their login pages. That should ensure that the API stays stable and that all required parameters are passed in.

Describe alternatives you've considered
n/a

Additional context
n/a

Prominent example: https://github.com/pellaeon/registration/blob/ef3afbab06165bf0e9bcdb4a85a055a71c5df05b/appinfo/app.php#L14-L17

[szaimen]

Is this still valid?

[ChristophWurst]

yes

[joshtrichards]

Fixed in #21850