python/iqm: add configurable message types to subscribe to

[ymorin-orange]

What:

Currently, the IQM subscribes to a wildcard topic (<ROOT>/#), which means it gets all the types of messages that are emitted on there: CAMs, DENMs, CPMs, etc...

However, some messages, like CAMs, are very high-bandwidth, and the IQM can't cope; instead, those messages can be handled by other means, like an MQTT bridge-to-self with topic rewriting.

We need a way to tell the IQM to restrict its subscriptions to a subset of the message types:

• configurable with a new config knob?
• can it be handled with existing filters on inqueue?

[ymorin-orange]

@mathieu1fb FYI

[ymorin-orange]

Check if the following filter could do the job, and at what cost:

[filter.no-cam]
in_prefix = {{inQueue}}/cam/
drop