[Form] allowed CallbackTransformer to use callable
minor #11576 [Form] allowed CallbackTransformer to use callable (issei-m)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11576).

Discussion
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[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) in CallbackTransformer, but its current constructor is defined argument type hinting for \Closure so i can't.

Commits
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e77022b [Form] allowed CallbackTransformer to use callable