Why:
These methods are used internally for the functionality of the library
and are subject to change. Therefore shouldn't be part of the public
interface.
This PR:
- Moves the ```register_strategy```, ```register_callback```,
```register_default_factories```, ```register_default_callbacks```
```strategies```, ```callback_names```
and ```strategy_by_name``` methods to the ```FactoryBot::Internal```
class.
- Deprecates the use of ```register_callback``` from the ```FactoryBot```
module.
Alejandro Dustet