Add support to capture custom version details

[smarr]

At the moment, we record only the most basic version details of the system.

It would be useful to add support for a general way to collect version information.
Since we don't know what's all relevant, a rebench configuration may want to define its own queries, commands to retrieve the version info, which is captured as part of the environmental details.

I am thinking here things like:

• versions of specific libraries and programs
• having a standardized way to have each executor report its version, or rather a configuration that tells us how to query for it
• having each build step have a list of things that allow us to query for the versions of dependencies