Add/Remove for FieldArrayType does not mark the form dirty.

[Jascen]

I'm submitting a ... (check one with "x")

[ ] bug report => search github for a similar issue or PR before submitting
[x] feature request
[ ] support request

Current behavior
Adding/Removing records in a custom FieldArrayType does not mark the dirty flag.

Expected behavior
Any user-driven actions will mark a form dirty. Per Angular's #16370, it is the action's responsibility to call markAsDirty().

Minimal reproduction of the problem with instructions
https://stackblitz.com/edit/angular-ferxhg

What is the motivation / use case for changing the behavior?
Every implementation of Field Array will have to override, call super, and then mark the form as dirty for both Add() and Remove().

[aitboudad]

added in v5.5.0 and thanks for the suggestion!