Add Sign in with Apple support

Hi folks! Nice to meet you!

I'm setting up Comentario as part of my blog redesign and it's been absolutely fantastic so far.

I do have one request though. My blog is predominantly about software development on Apple platforms so I feel like I have an obligation to offer Sign in via Apple as part of the third party identifier services.

Sadly, SIWA is a little more complex than usual, so setting it up with the generic OIDC provider doesn't seem possible.

Without having read too deeply into the source code, it looks like it should be relatively easy to slot in with the rest of the first class supported providers.

I can potentially try and post this as a merge request myself. Would that be okay?

Thanks again!

-Tim