Add support for Field Boundaries PV

[cholmes]

Is your feature request related to a problem? Please describe.
Sorta? I'd like to request field boundaries PV subscription, but I can't with request-pv

Describe the solution you'd like
Add field boundaries to the --var-type (and any other places it's needed)

Describe alternatives you've considered
Using another var-type and then hand modifying the json.

Additional context
It'd also be awesome if the client would provide feedback on the time ranges that are acceptable, as for field boundaries they are a bit narrow. But that's likely a lot more complexity and would likely mean having to keep the client in sync with the server.