OpenID: support fetching display name from Apple

This issue has been migrated from [#9375](https://github.com/matrix-org/synapse/issues/9375).

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AppleID sends the user's name in a `user` field in the oidc callback (see https://developer.apple.com/documentation/sign_in_with_apple/sign_in_with_apple_js/incorporating_sign_in_with_apple_into_other_platforms#3332115).

(This doesn't seem to be a standard field in [OIDC](https://openid.net/specs/openid-connect-core-1_0.html#AuthResponse) or [OAuth2](https://tools.ietf.org/html/rfc6749#section-4.1.2)