Add getUnitAgl rpc and update streamunits with same

[mobot-gh]

No description provided.

[YoloWingPixie]

LGTM

• Don't forget to update CHANGELOG Unreleased section

[dutchie031]

The code looks all good. No objections there.
I've left two comments, but as a bigger comment/discussion point is:
Do we want AGL as a more standardised transform/data point?

If we do:

• It shouldn't have it's own getUnitAGL (as it's in the getUnit as well)
• It should be part of the getTransform as it's then part of standardised 3d positioning information.

If we don't:

• It shouldn't be part of the stream data (if not standardised the client needs to request on demand)

Just thought it might be worth a thought