goose: add struct field injection

This makes options structs and application structs much simpler to
inject.

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

internal/goose/testdata/Struct/foo/goose.go

@@ -0,0 +1,7 @@
+//+build gooseinject
+
+package main
+
+//goose:use Set
+
+func injectFooBar() FooBar