[REQ][core] Add Mock feature to feature matrix

Created by: ybelenko

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

Feature matrix created by @jimschubert is great. I would love to see Mock feature there, because I've spent so many hours on the first release of PHP Data Mocker and it will be really disappointing if nobody notice it.

Describe the solution you'd like

I think Mock Server or just Mock feature fits modules/openapi-generator-core/src/main/java/org/openapitools/codegen/meta/features/GlobalFeature.java class.