Introduce the concept of profiles

[jgpruitt]

A profile is an optional "mode" to put the tuner in that tailors the recommendations for a particular workload. If no profile is specified, the tuner is run with a default profile which provides exactly the same behavior as before and corresponds to the typical timescaledb workload.

The only other valid profile defined at the moment is "promscale". With this commit, the promscale profile does not produce any results different from the default profile. This commit only provides the "plumbing" so that future commits can recommend different settings for the promscale profile.