Allow smoother restarts with third party integration

[Pugzy]

Updates to the PGM restart logic and ability for third-party integrations to modify restart behaviour.

This PR moves some heavy-lifting logic out of the RestartListener and in to the RestartManager. The last set of changes in #155 to the restart logic took things in a weird direction and muddled things up a bit.

RestartListener

• Now only listens to events and doesn't actually control logic of restarting.
• Holds a deferral if restarted whilst a match is playing and releases once complete.

RestartManager

• Becomes singleton hub of restarting, removes and replaces static function calls.
• Holds data on restart requests and starts the countdowns when required.
• Stores a list of referrals that come from the event.

RequestRestartEvent

• Deferring via this now adds to the RestartManager list of deferral.
• When removing a deferral via the event if no other deferrals exist a restart will be started.
• The event allows for a custom countdown to be used in the place of the regular restart one.

VotingPool

• Stores current poll and includes a PGM config option on if polls are allowed when a restart is queued.

SmoothRestartModule

• Can be ignored whilst PR is a draft, will be removed once ready.
• Using this to simulate how another plugin could interact with these changes.