Reconsider camera properties trigger metadata struct

[andy-sweet]

In acquire-project/acquire-core-libs#18, there was some discussion about the inability to express the trigger_wait output line/trigger in the properties metadata.

The CameraPropertiesTriggerMetadata struct captures three triggers (frame_start, acquisition_start, exposure) and allows to express which input/output lines these can be enabled on using a bit mask over the 8 possible lines.

One problem with this struct is that the trigger_wait output trigger is not captured by this struct, so its capabilities/metadata cannot be expressed. That can be easily fixed by adding a trigger_wait member to the CameraPropertiesTriggerMetadata struct.

But we may want to consider wider changes to this struct, so that it more closely mirrors the corresponding CameraProperties struct. For example, we could define a CameraPropertiesTriggerMetadata::input::acquisition_start bit-mask instead of CameraPropertiesTriggerMetadata::acquisition_start::input one.