Add add-on detection for premium transformers

[amontanez24]

Problem Description

As a user, it would be useful to be able to import enterprise transformers in the same way I import public ones.

Expected behavior

• Add add-on detection for transformers. There will be two types we need to support:
  1. Transformers to be added to already existing modules. An example of this is when we want to add premium transformers to the numerical module so that a user can do

  from rdt.transformers.numerical import PremiumNumericalTransformer

  2. Transformers in an entirely new module. An example could be if we wanted to add email transformers so a user could do

  from rdt.transformers.email import PremiumEmailTransformer

Additional context

• When creating new transformer add-ons, the entrypoint group name we use should indicate which case it falls under. A proposal could be to do something like rdt.new_transformers_module vs rdt.existing_transformers_module. In the former case we would have to add the module to rdt.transformers and in the latter case we would have to add all the new module in the add on to the existing sdtype transformer module (ie. rdt.numerical)
• We can use something similar to the add-on logic here