feat(oidc): add post route for /userinfo (#767)

easy two-liner to pass `oidcc-userinfo-post-header` test in conformance
suite.

internal/controller/oidc_controller.go

@@ -70,6 +70,7 @@ func (controller *OIDCController) SetupRoutes() {
 	oidcGroup.POST("/authorize", controller.Authorize)
 	oidcGroup.POST("/token", controller.Token)
 	oidcGroup.GET("/userinfo", controller.Userinfo)
+	oidcGroup.POST("/userinfo", controller.Userinfo)
 }
 
 func (controller *OIDCController) GetClientInfo(c *gin.Context) {

internal/middleware/context_middleware.go

@@ -24,6 +24,7 @@ var (
 		"GET /api/oidc/clients",
 		"POST /api/oidc/token",
 		"GET /api/oidc/userinfo",
+		"POST /api/oidc/userinfo",
 		"GET /resources",
 		"POST /api/user/login",
 		"GET /.well-known/openid-configuration",