Use OIDC base strategy for Apple strategy

[danschultzer]

Resolves #21

This uses OIDC rather than OAuth 2 as the base strategy for Apple so the JWT gets verified.

[danschultzer]

@stuartbain could you help out by testing this? Just set {:assent, github: "pow-auth/assent", ref: "apple-strategy-fix", override: true} in mix.exs, and check if your Sign in with Apple strategy still works. Unfortunately I have an expired developer membership 😞

Hopefully this just works🤞

[stuartbain]

@danschultzer I tested the apple-strategy fix branch as requested and my Sign in with Apple strategy still works. 👍

[danschultzer]

Thanks! 🙏