Support HTTP POST requests to /userinfo, aligning to OpenID Core sp…

…ecification (go-gitea#32578)

This PR adds support for the HTTP POST requests to `/userinfo` endpoint.
While the OpenID Core specification says both are supported and
recommends using HTTP GET.

ref: https://openid.net/specs/openid-connect-core-1_0.html#UserInfo

routers/web/web.go

@@ -551,7 +551,7 @@ func registerRoutes(m *web.Route) {
 		m.Post("/authorize", web.Bind(forms.AuthorizationForm{}), auth.AuthorizeOAuth)
 	}, ignSignInAndCsrf, reqSignIn)
 
-	m.Methods("GET, OPTIONS", "/login/oauth/userinfo", optionsCorsHandler(), ignSignInAndCsrf, auth.InfoOAuth)
+	m.Methods("GET, POST, OPTIONS", "/login/oauth/userinfo", optionsCorsHandler(), ignSignInAndCsrf, auth.InfoOAuth)
 	m.Methods("POST, OPTIONS", "/login/oauth/access_token", optionsCorsHandler(), web.Bind(forms.AccessTokenForm{}), ignSignInAndCsrf, auth.AccessTokenOAuth)
 	m.Methods("GET, OPTIONS", "/login/oauth/keys", optionsCorsHandler(), ignSignInAndCsrf, auth.OIDCKeys)
 	m.Methods("POST, OPTIONS", "/login/oauth/introspect", optionsCorsHandler(), web.Bind(forms.IntrospectTokenForm{}), ignSignInAndCsrf, auth.IntrospectOAuth)