endpoints: add Apple

[AlexCuse]

This adds endpoints for sign-in with Apple as defined in https://developer.apple.com/documentation/accountorganizationaldatasharing

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gopherbot]

This PR (HEAD: b947c2f24081c716c24cdc05b3a6d7ccb0c8eaa1) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/oauth2/+/555775.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/555775. After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps: A maintainer will review your change and provide feedback. See https://go.dev/doc/contribute#review for more info and tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be surprising to people new to the project. The careful, iterative review process is our way of helping mentor contributors and ensuring that their contributions have a lasting impact.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/555775. After addressing review feedback, remember to publish your drafts!