[REQ][php-nextgen] Put models in different directories

[ybelenko]

Is your feature request related to a problem? Please describe.

It would be great to place models like GetUsers200Response.php into Models/Responses instead of just Models. When client library becomes huge(100+ models) then some organisation into subfolders might be really helpful.

Describe the solution you'd like

Add an option to generator, when it's on the generator arrange models into Parameters, Responses folders, etc.

Describe alternatives you've considered

Tiny hint from maintainers could be great. Where I can find related code, to implement desired feature myself and make PR. @wing328

Additional context