Add support for handling HomeNSReq requests.

In this case, we return the (primary) NetID of the ChirpStack instance.
Supporting the HomeNSReq is useful in the case the roaming ChirpStack
instances are setup to resolve the servers using DNS. In case of OTAA,
the fNS resolves the JoinEUI to a hostname to which it must make the
HomeNSReq. This returns the HNetID, which then can be used to resolve
the hostname of the hNS to which the join-request must be forwarded.

backend/src/lib.rs

@@ -252,6 +252,10 @@ impl Client {
         Ok(ans)
     }
 
+    pub async fn home_ns_ans(&self, target_role: Role, pl: &HomeNSAnsPayload) -> Result<()> {
+        self.response_request(Some(target_role), pl).await
+    }
+
     pub async fn xmit_data_req(
         &self,
         target_role: Role,