The only thing left is to add an API endpoint for broadcasting
EventCustom events (other event types should not be emittible by
plugins, the use-case isn't there since plugins can already talk to
Zoraxy via the API).
Input to the endput should be a json-encoded `CustomEvent`
this design (as opposed to adding a Source field to the Event struct)
requires fewer changes to existing APIs while still supporting the two
primary cases for event sources:
1. an event that always has the same source can just return a hard-coded
string
2. an event that can come from multiple components (or from plugins) can
have a source field that gets returned by this function
