Add a generic SLO checker

The generic SLO (checker) aims to cover most simple types of SLOs and should be advertised as the default for new users trying out Theodolite. As our other checkers, it's running a small webserver that SLO check handles requests. This SLO is configurable by:

• a PromQL query
• a query aggregation function (possible values mean, median, max, min, p99, p99.9, ...
• a repetition aggregation function (same as above, used to aggregate the query aggregation results)
• a threshold (absolute value for now)
• an operator (to compare the aggregation result and threshold with, possible values lte, gte, lt, gt

Probably we can later get rid of the dropped-records SLO checker.

This feature should be backported to v0.6.0 as a feature preview.