feature: Add goctl completion (#1505)

* feature: Add `goctl completion`

* Update const

Co-authored-by: anqiansong <anqiansong@bytedance.com>

tools/goctl/completion/const.go

@@ -0,0 +1,9 @@
+package completion
+
+const BashCompletionFlag = `generate-goctl-completion`
+const defaultCompletionFilename = "goctl_autocomplete"
+const (
+	magic = 1 << iota
+	flagZsh
+	flagBash
+)