feat: Add extension point for CanJoinMatchmakeSession

[ashquarky]

Changes:

Adds the option for games to override CanJoinMatchmakeSession with their own version if needed. This is useful for, at least:

• Minecraft: Wii U Edition, which doesn't use NEX ParticipationPolicy at all and instead uses a combination of search attributes and application buffers to determine join eligibility. This is I guess done to allow "Friends of friends" matchmaking on the client; but we'd like to check it serverside too for security reasons.
• Splatoon's Splatfests, which involve matchmaking the second team into a room with OpenParticipation false - the current check forbids this. Presumably this is to prevent random individuals matching into a room without a whole team alongside them, which would make the room unplayable (any legit teams would find there aren't enough vacant participants)

I have a sample of what Minecraft's matchmaking looks like here. I'm not too sure if you like the design here, since it encourages reaching into the database directly (as I did in the minecraft diff and database.FindGatheringByID to get the match participants).

• I have read and agreed to the Code of Conduct.
• I have read and complied with the contributing guidelines.
• What I'm implementing was an approved issue.
• I have tested all of my changes.

[DaniElectra]

The Minecraft use case seems fair enough.

The Splatfest issue sounds like something may not implemented correctly, like the gidForParticipationCheck which currently isn't used. Thus, the override probably shouldn't be required there (unless proven otherwise)

[DaniElectra]

LGTM