refactor: add config file for benchmarks

[ielashi]

This PR is another step towards transforming our benchmarking logic into a general-purpose tool that others can use.

Rather than hard-coding everything in benchmark.rs, the configuration is moved into a config file called bench.yml.

The eventual goal is to evolve the current benchmarking work into a tool that works like this:

1. User defines a configuration file (e.g. bench.yml)
2. User installs our benchmarking tool (e.g. cargo install canbench)
3. User can now run benchmarks using cargo (e.g. cargo canbench)

Note that users won't need to write any code and the file benchmark.rs will soon be completely removed.

[AlexandraZapuc]

Lgtm!