goose: support provider cleanup functions

Documented and updated appropriate providers to reflect.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/Cleanup/foo/goose.go

@@ -0,0 +1,8 @@
+//+build gooseinject
+
+package main
+
+//goose:use Foo
+//goose:use Bar
+
+func injectBar() (*Bar, func())