Add worker tagging (#862)

This allows workers to have tags assigned to them, which can then be filtered to control which targets they are allowed to serve.

internal/proto/local/controller/storage/target/store/v1/target.proto

@@ -53,6 +53,10 @@ message TargetView {
   // Maximum number of connections in a session
   // @inject_tag: `gorm:"default:null"`
   int32 session_connection_limit = 110;
+
+  // A boolean expression that allows filtering the workers that can handle a session
+  // @inject_tag: `gorm:"default:null"`
+  string worker_filter = 120;
 }
 
 message TargetHostSet {
@@ -124,4 +128,11 @@ message TcpTarget {
     this: "SessionConnectionLimit"
     that: "session_connection_limit"
   }];
+
+  // A boolean expression that allows filtering the workers that can handle a session
+  // @inject_tag: `gorm:"default:null"`
+  string worker_filter = 120 [(custom_options.v1.mask_mapping) = {
+    this: "WorkerFilter"
+    that: "worker_filter"
+  }];
 }