controller to transform and forward commands

[ssube]

Summary

Triggering one command because of another command (or message) is the last major component to self-updating (#313, #544). This controller has a few needs, met by existing services:

• conditional execution, handled by filters
• payload modification, handled by transforms

Thanks to that existing code, the only new functionality in this is a controller is executing a new command.

Scope

• figure out appropriate name
• execute-command controller
• docs
• schema
• tests

Use Case

• approving an MR/PR after receiving check status, excluding WIP:
• merging an MR/PR after receiving approval, based on author