Grouped dropdown support (Prime NG)

[Dmitry Gorelenkov]

I'm submitting a

[ ] Bug / Regression
[x] Feature Request / Proposal

I'm using

NG Dynamic Forms Version: `17.0.0`

[ ] Basic UI
[ ] Bootstrap UI  
[ ] Foundation UI
[ ] Ionic UI
[ ] Material  
[ ] NG Bootstrap
[x] Prime NG

Description

Would be nice to have support for grouped dropdown (https://primeng.org/dropdown#group)
right now only simple values are possible.