Extract Reactive Operations to a Ruby Gem

I built a proof of concept of the reactive part of triage-ops using AWS Lambda. It was quite challenging picking out the bits I needed/wanted and would be a maintenance nightmare trying to integrate changes made to this project.

To make the reactive part of this project more reusable, we should consider making it a Ruby Gem (as has been done with the non-reactive bit, gitlab-triage).

Linking #785 and gitlab-org/quality/engineering-productivity/team#10 as interesting and may be worth considering changing the architecture more substantially (building from the ground up, picking and choosing the best bits from the existing framework).

/cc @kwiebers