router: add upgrade method

This is just an Async GET method, but semantically
different, so it gets its own router method.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/api_schema/router.rs

@@ -234,6 +234,10 @@ impl Router {
         self
     }
 
+    pub fn upgrade(mut self, m: ApiAsyncMethod) -> Self {
+        self.get = MethodDefinition::Async(m);
+        self
+    }
 
     pub fn delete(mut self, m: ApiMethod) -> Self {
         self.delete = MethodDefinition::Simple(m);