Framework for defining which benchmarks are Arrow

[nealrichardson]

We have two related challenges:

• Some parameters we want to test are only relevant for Arrow. Some of these, like whether to return an arrow Table or data.frame when reading a file, may be specific to a single benchmark. Others, like environment variables that only affect Arrow, should be controllable at the global level. We don't want to run non-arrow benchmarks parametrized by arrow-only environment variables--that's wasteful/slow.
• When running benchmarks continuously (on every commit), we only want to run the Arrow benchmarks--no point wasting electrons on other people's (unchanged) code.

What if we added a function (arrow_params()?) similar to default_params that each benchmark can register that indicates which param combinations involve arrow? We can use that in whatever run_all() function we add for conbench, and we can also use it to only add variations for arrow env vars to the combinations that involve arrow.