feat: add discovery

Signed-off-by: kjuulh <contact@kjuulh.io>

crates/churn/proto/churn/v1/churn.proto

@@ -5,6 +5,7 @@ package churn.v1;
 service Churn {
   rpc GetKey(GetKeyRequest) returns (GetKeyResponse);
   rpc SetKey(SetKeyRequest) returns (SetKeyResponse);
+  rpc ListenEvents(ListenEventsRequest) returns (stream ListenEventsResponse);
 }
 
 message GetKeyRequest {
@@ -23,3 +24,11 @@ message SetKeyRequest {
   string value = 4;
 }
 message SetKeyResponse {}
+
+message ListenEventsRequest {
+  string namespace = 1;
+  optional string id = 2;
+}
+message ListenEventsResponse {
+  string value = 1;
+}