add istioctl completions (#8343)

This program uses the Cobra framework for argument parsing and completion generation.
Just source the completions supplied by upstream.
This works around "go install" not being able to install completions files (only binaries).

CHANGELOG.rst

@@ -99,6 +99,7 @@ Completions
 ^^^^^^^^^^^
 - Added completions for:
 
+  - ``istioctl`` (:issue:`8343`)
   - ``black`` (:issue:`8123`)
   - ``exif`` (:issue:`8246`)
   - ``gping`` (:issue:`8181`)