add Azure AD B2C endpoints

[hsnkorkmaz]

It is very common to mix up Azure AD and Azure B2C. Since endpoints are not the same, I implemented a function to get Azure AD B2C endpoints according to the documentation below.

https://docs.microsoft.com/en-us/azure/active-directory-b2c/tokens-overview#endpoints

[gopherbot]

This PR (HEAD: e3b8814) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/387454 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

[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/387454.
After addressing review feedback, remember to publish your drafts!