- Adds `usagetypes.UnknownEventTypeError` type, which is returned by
`ParseEventWithType`
- Changes `ParseEvent` to not be a generic function since it doesn't
really need it
- Adds `User-Agent` to tallyman requests
Not used in coderd yet, see stack.
Adds two new packages:
- `coderd/usage`: provides an interface for the "Collector" as well as a stub implementation for AGPL
- `enterprise/coderd/usage`: provides an interface for the "Publisher" as well as a Tallyman implementation
Relates to https://github.com/coder/internal/issues/814