kdaectrl: expose vetos as PVs and set veto mask

[rerpha]

realised we didn't have a ticket for this so creating one.

kdaectrl needs to be able to serve PVs that set vetoes and read the status of them.
This ticket is to implement that behaviour.

It needs to emit a vc00 blob to a vetoes topic (which we should document in the list of topics etc. )