2022-May-30 - Prerelease - 2.6.817

Decouple headless code (#10530)

* Reduce usage of HeadlessGameServer#headless

* Remove pointless if check

* Move CLI setup to HeadlessGameRunner

* Reduce usage of static server instance

* Drop redundant static qualifiers

* Fix typos

* Avoid side-effects in constructor

* Move headless check to GameRunner

* Move headless related classes to headless project

* Address PR comments