Add support for unlimited and easier recursion for FormGroups

[intellix]

Is your feature request related to a problem? Please describe.
Trying to create a Condition/Expression builder using Formly is extremely painful due to the requirement to manually do recursion and pollution of the model.

Describe the solution you'd like
I want to be able to do recursion within FieldGroups without having to manually re-create FieldGroups and pollute the model

Describe alternatives you've considered
I've created my form so far and I'm manually having to clean it up afterwards and it's limited to an amount of recursion that I specify.

Additional context
https://stackblitz.com/edit/angular-rhagg9-1bfpvt?file=src%2Fapp%2Futils.ts