Specify DBE mask with PVA subscriptions

[mdavidsaver]

One difference between CA and PVA protocols is that PVA does not have the same native conception of DBE, in particular there is no direct analogy of DBE_VALUE vs. DBE_ARCHIVE. With PVA, the pvRequest blob is intended for this sort of modifier.

QSRV2 looks for record._options.DBE in a monitor pvRequest, this can either by a string, following the parsing behavior of the old caChannel from pvAccessCPP, or an integer bit mask. In either case, VALUE, ARCHIVE, and ALARM can be specified.

https://github.com/epics-base/pvxs/blob/ff1d6510cbf84229f7018eacf4008ecd85dce565/ioc/singlesource.cpp#L108-L118

That said, I see that PVXS has no test coverage of this feature, and perhaps unsurprisingly it doesn't work (epics-base/pvxs#97).

So I intend this ticket as a placeholder for eventually getting DBE handling sorted out on both ends.

@kasemir fyi.