Allow Benchmarking framework to exercise Git operations

The RPCs that handle Git operations PostUploadPackWithSideChannel, PostReceivePack, SSHUploadPackWithSidechannel, SSHReceivePack are not currently exercised by the benchmark.

Given these heavy hitting Git operations are of great interest to customers, let's develop a way of exercising these RPCs as a proxy for the kind of load that git fetch and git push would put on Gitaly.