This is a good starting point, but I want to highlight that this doesn't support custom type field mappers. I'm afraid if we did this, in few months someone will complain they can't define custom mapper through configuration. Any suggestions to avoid that? Other than that, this PR is completely missing tests of course. And DefaultTypeFieldMapper service should be defined in orm.xml file. DoctrineExtension will then only reference to it.

I see your point. This would be the easiest for users to configure, but not the most flexible.
Another solution I see is allowing one to simply set a TypedFieldMapper service.

dbal:
    orm:
        typed_field_mapper: '@Doctrine\ORM\Mapping\DefaultTypedFieldMapper'

Which then needs to be configured in services.yaml

If that's the preferred way, I can work on that (including tests)

Let's go with this: For now we don't need support for something else than DefaultTypedFieldMapper. In future if there is a need for custom field mapper, we will add another key (like the one you proposed) and disallow combining it with typed_fields keys in this PR. So this PR is quite feature complete (at least for now), but please add tests.