Make it easier to extend the supported attribute types

[lyz-code]

As a user, I want to easily extend the available mappers of the pydantic model attribute types, for example, if there is a class that has an attribute type not supported by pydantic-factories and involves third-party libraries like this case, the extension of the mappers should be done on the user side.

Right now to do this the user has to subclass ModelFactory, override the method and extend the base value. This has the downsides that:

• As the returned value of get_provider_map is generated on the return statement, to override the method the user has to copy and paste the whole dictionary potentially being outdated on future pydantic-factories releases.
• It's not documented how to do this.
• It's not easy for the user to do all this.

What I'm proposing is that ModelFactory has an internal attribute settable by the user where she can define the additional mappers, for example to give support to FakeDateTime the user's code would be:

from freezegun.api import FakeDatetime

class CustomModelFactory(ModelFactory):
    additional_mappers = { FakeDateTime: cls._get_faker().date_time_between }

We'll have to tweak get_provider_map so that it merges the default mapper dictionary with the user's

[Goldziher]

I have no problem with this proposal you're welcome to add a PR. Please note the following though:

1. we use the dunder pattern for factory variables, i.e. __additional_mappers__ instead of additional_mappers
2. you could contribute documentation regarding how to subclass the factory, which will resolve the issue
3. you can also subclass the method get_mock_value

[lyz-code]

Mocking get_mock_value makes more sense, I'll add the docs once I've got a working example, thanks!

[lyz-code]

Done!