Missing array rate field needed on PCDSAreaDetectorTyphos

[ZLLentz]

Current Behavior

The only rate field present on this detector is event_rate, for the PCDSAreaDetectorTyphosTrigger variant, and this field is config so it isn't seen by default in the screen. This particular field gives us the expected rate of the incoming triggers.

Expected Behavior

James has asked if we can add the actual rate too, with the actual rate being shown in the typhos screen.
The PV suffix for this is ArrayRate_RBV and is not used in any of our existing classes. It should be added at kind=normal.

Context / environment

It'd be useful to see this number as a quick spot-check that the camera is running.

Suggested Solution

Get some buy-in and then add one PV