[frdb] Add BUILD.gn file. (#17673)

Adds fuchsia_remote_debug_protocol gn file.

packages/fuchsia_remote_debug_protocol/BUILD.gn

+import("//build/dart/dart_library.gni")
+
+dart_library("fuchsia_remote_debug_protocol") {
+  package_name = "fuchsia_remote_debug_protocol"
+
+  # Can be left empty as analysis is disabled.
+  sources = []
+
+  disable_analysis = true
+
+  deps = [
+    "//third_party/dart-pkg/git/flutter/packages/flutter",
+    "//third_party/dart-pkg/git/flutter/packages/flutter_test",
+    "//third_party/dart-pkg/pub/json_rpc_2",
+    "//third_party/dart-pkg/pub/meta",
+    "//third_party/dart-pkg/pub/process",
+    "//third_party/dart-pkg/pub/web_socket_channel",
+  ]
+}