Removed TransitionToActiveEvent

[nassosterz-ms]

What this PR does / why we need it:

• Solves the locked main thread issue when server is on Standby.
• Replace TransitionToActiveEvent object with 2 new Delegates: OnServerActive and OnGameServerInitializationComplete
• OnGameServerInitializationComplete allows user to add custom code when game server transitions from Initialization to
  StandBy
• OnServerActive allows user to add custom code when game server transitions from StandBy to Active

Special notes for your reviewer:

• Has been tested with UE 4.27.1 and UE5 early access

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

[stkrwork]

UGSDKReadyForPlayersAction also needs to be adapted to work with the delegates.