Add score token to BeginPlaySession()

[smoogipoo]

This is a breaking change, and will require an osu-server-spectator build with ppy/osu-server-spectator#153

This will allow osu-server-spectator to lookup the score ID from the score token.

	Old Server	New Server
Old Client	🟢	🔴
New Client	🔴	🟢

[bdach]

I'm not entirely sold on the decision of placing the token in SpectatorState like this if I'm honest. The structure is being broadcast to every other connected client. While there may be requisite security checks web-side, broadcasting it like that seems like... an unnecessary risk, I guess? May be just my paranoia talking but I'm a strong believer in defence in depth.

I do realise that not broadcasting it is probably going to be quite annoying (separation of data structures, etc.). You could just blank the token server-side for the time being.

[smoogipoo]

Given that #21587 is a breaking change anyway, I've taken the liberty of moving the score token to the BeginPlaySession invocation here, making this PR now also a breaking change.

Now requires ppy/osu-server-spectator#153 for testing/deployment.

[bdach]

Seems simple enough