-api should broadcast it's capabilities for clients

[fnoop]

Related to #204 - should -api send it's current modules/capabilities so that clients will know what they can/can't request?
For example, if -api doesn't have access to (currently) mavros, then it can't send mavlink/mavros data. The client should then know that so it doesn't request it. But (base) maverick data should still be available at all times, so it should be able to send/request that.

[SamuelDudley]

@fnoop this is possible via a schema introspection request from the client, which provides the client with the full schema / what it can request.
I will add loaded modules to the status.

[SamuelDudley]

Advertise loaded modules and notify changes to schema

[fnoop]

Client work tracked in goodrobots/maverick-web#151

[fnoop]

We could add an -api parameter to choose which kind of zeroconf to use - either an existing zeroconf service or start a fresh one. So if there's an existing avahi/whatever running on the system we can register with that, otherwise we can start a new service.
It looks like python-zeroconf starts a new service (which clashes with avahi) and won't co-exist.
https://github.com/jstasiak/python-zeroconf

We can add services to avahi either by placing an xml definition in /etc/avahi/services, or (better) by communicating with it over dbus:
https://github.com/lathiat/avahi/tree/master/avahi-python
https://stackoverflow.com/questions/3430245/how-to-develop-an-avahi-client-server
https://en.wikipedia.org/wiki/Zero-configuration_networking

python-zeroconf and avahi both listen on udp port 5353 to respond to mdns requests over multicast. We (-api) don't actually want to serve mdns, but rather just the service discovery portion (and just a single entry at that). So logically we don't want to run zeroconf through -api unless we absolutely have to, rather just register a service with an existing zeroconf service.

It might be easier to just have avahi as a zeroconf dependency for -api, and just add support to register a new service with avahi?