[Benchmarking] Support for on-demand and automated benchmarks

[asfimport]

We want to be able to request for a benchmark comparison in a PR against master. This should be triggered via a github comment.

The automated benchmarks would track the master branch and be triggered either on each merge or nightly.

Reporter: Francois Saint-Jacques / @fsaintjacques

Related issues:

• [C++] Implement benchmark comparison between two git revisions (is blocked by)
• [Benchmarking] Performs a benchmark run with archery (is blocked by)
• [Benchmarking] Add portable machine and benchmark context detection (is blocked by)
• [C++] Whitelist benchmarks candidates for regression checks (is blocked by)
• [Benchmarking] Performance Benchmarking Dashboard (is blocked by)

_{Note: This issue was originally created as ARROW-5070. Please see the migration documentation for further details.}

[pitrou]

The conbench machinery now allows us to do that, closing.