[Bridge][Doctrine][Form] Add entity manager instance support for em option

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10157
| License       | MIT
| Doc PR        | symfony/symfony-docs#4336

This PR allows to pass an explicit entity manager instance as em option.