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/StructOptionalField/foo/foo.go

@@ -0,0 +1,23 @@
+package main
+
+import "fmt"
+
+func main() {
+	fb := injectFooBar()
+	fmt.Println(fb.Foo, fb.OptionalBar)
+}
+
+type Foo int
+type Bar int
+
+//goose:provide Set
+//goose:optional OptionalBar
+type FooBar struct {
+	Foo         Foo
+	OptionalBar Bar
+}
+
+//goose:provide Set
+func provideFoo() Foo {
+	return 42
+}