go api: message content typing - broadcaster to own package

[benw10-1]

Initial scope was basically converting MessageContent from a map[string]interface{} to a concrete type.
This accomplishes two things:

1. Makes more clear the usage path for the MessageContent field. You can look at the type and see exact ref. to the STR mod
2. For stuff we are not reading and just forwarding, we don't have to marshal into a map then remarshal it to bytes when we are broadcasting

The scope expanded a tiny bit because I had to change the function signatures for some of the broadcaster stuff.

See (#22) for API handler test initial implementation, this branched is based off of the API test branch. Needed some realistic tests as we need to ensure we are not breaking anything when the data entry-point is the Gin app.

Going to make a follow up PR for the Twitch PubSub endpoint stub and integration.
Going to make a follow up PR for Redis in CI pipeline for better DB testing.

Hopefully the addition of these types, the test-data, and tests on the endpoints should make failures less likely.