Support HTTP POST requests to `/userinfo`, aligning to OpenID Core specification

[marcellmars]

This PR adds support for the HTTP POST requests to /userinfo endpoint. While the OpenID Core specification recommends using HTTP GET, at least MinIO uses HTTP POST in their OIDC login flow.

OpenID Core:

• 5.3. Userinfo Endpoint:

  The UserInfo Endpoint MUST support the use of the HTTP GET and HTTP POST methods

• 5.3.1. Userinfo Request:

  It is RECOMMENDED that the request use the HTTP GET method and the Access Token be sent using the Authorization header field.

I tested this manually with a local MinIO instance, and it successfully allows their [x] Claim User Info setting to be checked. Without this change, it fails with a Method not allowed error from the IdP.

[GiteaBot]

I was unable to create a backport for 1.22. @marcellmars, please send one manually. :tea:

go run ./contrib/backport 32578
...  // fix git conflicts if any
go run ./contrib/backport --continue