RPC endpoint to request a rewards root at a snapshot height.

[seanmcgary]

Add a new RPC endpoint to the proto definitions and stub out the handler in the server to generate reward amounts.

In order to reach parity with our current infrastructure, we need to generate rewards based on a snapshot date rather than a block height.

message GenerateRewardAmountsRequest {
    int64 snapshot = 1; // unix timestamp representation of the snapshot
}

message RewardAmount {
    string earner = 1;
    string token = 2;
    int64 snapshot = 3; // unix timestamp representation of the snapshot
    string cumulativeAmount = 4;
}

message GenerateRewardAmountsResponse {
    repeated RewardAmount rewards = 1;
}


rpc GenerateRewardAmounts(GenerateRewardAmountsRequest) returns GenerateRewardAmountsResponse {
    option (google.api.http) = {
      post: "/v1/reward-amounts"
      body: "*"
    };
}