apple: Add new endpoint of Apple Sign-in

[cixtor]

Apple’s new “Sign-in with Apple” service announced during WWDC 2019 is based on OAuth 2.0 and OIDC (OpenID Connect) plus JWT for the application credentials. The public keys to verify the JSON Web Token (JWT) signature are available here. We can validate the authorization grant code with Apple to obtain tokens or validate an existing refresh token using this endpoint. And the documentation for the token response is available here.

[gopherbot]

This PR (HEAD: 0daeb82) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/181478 to see it.

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[gopherbot]

Message from Brad Fitzpatrick:

Patch Set 1:

Per the policy documented at https://github.com/golang/oauth2#policy-for-new-packages we no longer accept mini packages for just endpoints.

Feel free to add this to a new "endpoints" package, so the symbol would be "endpoints.AppleSignIn" for example.

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[gopherbot]

This PR (HEAD: b3ec657) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/181478 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info