[milo] setup queryUniqueTestVariants RPC in the frontend
[milo] setup queryUniqueTestVariants RPC in the frontend

R=meiring, nqmtuan

Bug: 1278546
Change-Id: If2b0bfc33463d14a58f06d0151a3be4a4f0fea8a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3399368
Reviewed-by: Patrick Meiring <496ae7b7898d641db15cbb7f3340e9390316ae10@google.com>
Reviewed-by: Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Auto-Submit: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

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