Add a generic "latency" slo plugin

[jmleddy]

I think it'd be conceptually easier if we had a plugin that just did latency for you, in a way similar to the istio ones, but it could be based on any metric. Rough completely unvalidated outline yaml example below:

config:
  metric: web_request_seconds
  filter: app="nginx",region="va",env="prod"
  bucket: 100

This would just do the web_request_seconds_bucket and web_request_seconds_count calculations with any of the out of tree latency plugins, but in a much more generalizable way so anyone can use it instead of resorting to creating their own plugin. This is also one of the only things that pyrra had out of the box that sloth lacks.