Support External Login Providers for Membership

[cleversolutions]

While discussing #10656 @Shazwazza mentioned that external login providers for member are not likely to be supported in 9.0.0, but will likely be available in a later 9.x.

Unfortunately external login providers for members will probably not make it to 9.0.0 release since that feature isn't actually part of the CMS in v8. My own library called UmbracoIdentity filled this gap for v8 but that will be retired since this functionality will become directly available in v9.x at some stage.

That said, it should be more than possible to make this work on your own since Umbraco 9.0.0 comes with all of the identity requirements for external logins: IMemberSignInManager/MemberSignInManager, IMemberManager/MemberManager. Most of the concepts can be ported over from the aspnetcore examples for external logins (which are based on sign in manager and user manager, but are backed by EFCore providers... but the underlying provider will not matter).

Originally posted by @Shazwazza in #10656 (comment)

It seems to me that adding support for external login providers to Umbraco would be marginally more effort than adding it to a specific project, but would benefit everyone. So let's do something that makes everyone (who needs external login for members) happy, and start a discussion on adding this feature in 9.x 😄