Add IdentifyGooglePlayGames API

[tudddorrr]

No description provided.

[claude]

PR Review: Add IdentifyGooglePlayGames API

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Major: Missing identity parameter for account linking

IdentifySteam supports an optional identity parameter for account linking but IdentifyGooglePlayGames does not:

IdentifySteam signature:
public async Task IdentifySteam(string ticket, string identity = "")

IdentifyGooglePlayGames signature (new):
public async Task IdentifyGooglePlayGames(string authCode)

If account linking needs to be supported for Google Play Games later, adding identity would be a breaking change. Adding it now as an optional parameter costs nothing and keeps the API surface consistent:

public async Task IdentifyGooglePlayGames(string authCode, string identity = "")
{
if (string.IsNullOrEmpty(identity))
{
await Identify("google_play_games", authCode);
}
else
{
await Identify("google_play_games", identity + ":" + authCode);
}
return Talo.CurrentPlayer;
}

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No other issues found. The implementation is clean and consistent with the existing pattern.