Flutter attach (#18677)

Add `flutter attach` command.

This command allows Flutter to connect to applications that haven't been launched using `flutter run`.

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 08a488c..148faf5 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -269,6 +269,12 @@
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
 
+  flutter_attach_test:
+    description: >
+      Tests the `flutter attach` command.
+    stage: devicelab
+    required_agent_capabilities: ["linux/android"]
+
   # iOS on-device tests
 
   flavors_test_ios: