goose: add optional provider inputs

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

internal/goose/testdata/OptionalMissing/foo/foo.go

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectBar())
+}
+
+type foo int
+type bar int
+
+//goose:provide
+//goose:optional f
+func provideBar(f foo) bar {
+	return bar(f)
+}