Add DownloadLeaderboardEntriesForUsers method in SteamUserStats

[TSRBerry]

Somehow missed that the proto file already contains the definition for this.

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Resolves #1619

[xPaw]

Did you test this?

[TSRBerry]

Yeah, it works fine!

Some output from a small demo script I wrote

Getting leaderboard for: RANDOM
FindLeaderboard failed: OK / 0
Getting leaderboard for: SIDEQUEST_FISHERMAN_SUPLEX
Leaderboard data for: SIDEQUEST_FISHERMAN_SUPLEX
        SteamID: [U:1:225391317] | GlobalRank: 75223 | Score: 88351

Getting leaderboard for: SIDEQUEST_ATTITUDE_ADJUSTMENT
Leaderboard data for: SIDEQUEST_ATTITUDE_ADJUSTMENT
        SteamID: [U:1:225391317] | GlobalRank: 48678 | Score: 46168

Getting leaderboard for: TUT_TRIPWIRE
Leaderboard data for: TUT_TRIPWIRE
        SteamID: [U:1:225391317] | GlobalRank: 119110 | Score: 36800

Getting leaderboard for: GRID_BOSS_TEMPLE2
FindLeaderboard failed: OK / 0
Getting leaderboard for: TUT_BOMBJUMP
Leaderboard data for: TUT_BOMBJUMP
        SteamID: [U:1:225391317] | GlobalRank: 33128 | Score: 12567
        SteamID: [U:1:225913021] | GlobalRank: 65630 | Score: 14173
        SteamID: [U:1:346502788] | GlobalRank: 434678 | Score: 45530