v2alpha1: Add Highest rpc to Activation service

[kacpersaw]

Description

V2 API lacks the Highest ATX method, which is present in V1.

In V1, the Highest method is defined as follows:

message HighestResponse {
    Activation atx = 1;
}

// Highest returns the atx id with the highest tick count.
rpc Highest(google.protobuf.Empty) returns (HighestResponse);