proto: backport HttpProxyRequest/Response (tags 27, 28)

Backports the HeaderEntry / HttpProxyRequest / HttpProxyResponse messages
and the corresponding union tags from upstream rustdesk/hbb_common
@87b11a7 onto the OSS server's pinned commit @83419b6 — wire-compatible
with the client-side encoder in rustdesk's src/common.rs::tcp_proxy_request.

Cherry-pick over a full submodule bump because the upstream commit pulls
newer transitive deps (notably tokio) that risk breaking axum 0.5 in the
hbbs HTTP layer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

protos/rendezvous.proto

@@ -170,6 +170,29 @@ message HealthCheck {
   string token = 1;
 }
 
+// Backported from upstream rustdesk/hbb_common @ 87b11a7 so the OSS hbbs
+// can implement the HTTP-over-rendezvous fallback the client uses when
+// OPTION_USE_RAW_TCP_FOR_API=Y. Wire-compatible with the client; only the
+// three message types and tags 27/28 are added.
+message HeaderEntry {
+  string name = 1;
+  string value = 2;
+}
+
+message HttpProxyRequest {
+  string method = 1;
+  string path = 2;
+  repeated HeaderEntry headers = 3;
+  bytes body = 4;
+}
+
+message HttpProxyResponse {
+  int32 status = 1;
+  repeated HeaderEntry headers = 2;
+  bytes body = 3;
+  string error = 4;
+}
+
 message RendezvousMessage {
   oneof union {
     RegisterPeer register_peer = 6;
@@ -193,5 +216,7 @@ message RendezvousMessage {
     OnlineResponse online_response = 24;
     KeyExchange key_exchange = 25;
     HealthCheck hc = 26;
+    HttpProxyRequest http_proxy_request = 27;
+    HttpProxyResponse http_proxy_response = 28;
   }
 }