Signatures not generated for models using new ActiveRecord::DelegatedType

[cguess]

Describe the bug:
When using the new DelegatedType feature in Rails 6.1 there are not signatures generated for delegated types from a base class, causing errors since it can't detect that the method is available.

Steps to reproduce:
Follow the steps as outlined in the Rails documentation to create a delegated type, for instance let's say Item is the base class and Book is the delegated type. Then run bundle exec rake rails_rbi:models to generate everything normally.

From there, in a controller write Item.books (which will run fine) and it will report "Method books does not exist on T.class_of(Item)" when checking level is more than false.

Expected behavior:
I would expect definitions to be read in this and be generated so that an array of Book would be expected to respond.

Versions:

• Ruby: 3.0.1
• Rails: 6.1.3.2
• Sorbet: 0.5.6424
• Sorbet-Rails: 0.7.4