goose: add interface binding

An interface binding instructs goose that a concrete type should be used
to satisfy a dependency on an interface type. goose could determine this
implicitly, but having an explicit directive makes the provider author's
intent clear and allows different concrete types to satisfy different
smaller interfaces.

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

internal/goose/testdata/ImportedInterfaceBinding/foo/foo.go

@@ -0,0 +1,5 @@
+package foo
+
+type Fooer interface {
+	Foo() string
+}