Allow external specification of channel types in STM generation

[MattWindsor91]

Currently, when we're generating STMs, we always infer the channel types (and do it locally, which means that different tests can infer different types). This is clumsy and won't always give us the result we need, and so it would be much better if we could allow the user to specify the types of channels in a side-channel such as an XML file or name: type list.

[MattWindsor91]

In discussion with @pefribeiro it's become evident that this would be useful to specify channels/topics that are not covered by any tests, too.

The inference happens at the test tree -> STM level (and I think that contrary to what I wrote in March it might be global with unification now?), so I'm unsure what the best side-channel would be for this.