Epic: Notifications Subsystem

[dannykopping]

We will be building a notification subsystem into Coder which will facilitate the sending of manual & automatic messages.

We're going to start off by building a low-fidelity vertical slice of the functionality to validate a few ideas. This version will not be merged into main necessarily; it's more to smoke out any flaws / oversights in the design.

Initial spike

• create terraform resource referencing coder_agent metadata keys for rule-based evaluations
• create database entities for notification "templates", "instances", and "conditions"
• create basic evaluation engine to create notification "instances" from "template" once coder_agent metadata value crosses threshold (or some other condition becomes true)
• create and run n in-process notification consumers (reading from the database queue)
  • start with n=1 initially, but stretch goal to have n>1 working (to sniff out lock contention problems)
• implement routing of notification instances to the matching workspace(s)
• enable basic notification display in Coder UI
• (stretch) enable notification display in CLI
• (stretch) enable notification display in VSCode
• (stretch) enable notification display in JetBrains

[dannykopping]

We're rescoping this project to deal with system-generated notifications instead.
I will link to the new milestone once created.

[dannykopping]

https://github.com/coder/coder/milestone/35