Adapt idiomatic container configuration

[aevitas]

Currently, serviceContainer.ts hard-wires a bunch of dependencies, which is far from perfect.

Ideally, we'd have the usage of malown be something along the lines of:

import { service } from "malown";

service.addSlack({ slackOptions });
services.addSendGrid({ sendGridOptions });

Which makes it a lot easier for end users to plug and play their own messaging providers without having to wire up the DI container manually.