Allow making raw HTTP POST requests to the authz server

[misberner]

This PR adds functionality to make token-style HTTP requests to an arbitrary endpoint of the authz server, instead of just for retrieving tokens.

Use case: the main motivation was to support for OAuth2 token revocation per RFC7009. The token revocation endpoint requires client credentials, and implementing it outside of this library requires effectively re-implementing the auth style auto-sensing & caching.

This seemed the most convenient way of leveraging the library for token revocation. If desired, I can also make this less general to only support token revocation, but a.o.t. this would require figuring out the revocation URLs for all supported providers, which is outside my current time budget.

[gopherbot]

This PR (HEAD: d2a14ae) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/277272 to see it.

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[gopherbot]

This PR (HEAD: 0b4d90a) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/277272 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

[janisz]

Fixes:

• proposal: x/oauth2: allow making raw post requests to server go#72028

[octavio12345300]

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