[Beginning of shutdown] PVA Site gateways

As a developer I would like to be able to access PVA PVs defined in the halls from my developer machine.

To do this we need a p4p site gateway.

### Acceptance criteria
- From a machine in [r3/r55/r80/r2] I can access PVA PVs that are in another building (all permutations)

### Notes
- Site gateways run on a `control-svcs` - credentials in usual place
- Be careful with firewall config, the firewall is necessary to prevent loops (& ensuing broadcast storms) on the *existing channel access gateways*
* This ticket should be done at the beginning of a shutdown to minimise risk to instruments of fiddling with firewall configs
- Spun out from https://github.com/ISISComputingGroup/IBEX/issues/8309

### Time in Planning
01:22:46 08/08/24