[Form] allowed CallbackTransformer to use callable

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

I want to use the callable(i.e. inner class methods) at CallbackTransformer, but its current constructor is defined argument type hinting so i can't.