CAChannel Wrapper: optionally use p4p instead of CaChannel

[rerpha]

As a developer I would like anything that uses our CaChannel wrapper to be able to "speak" PV access. as we have an abstraction layer this should be somewhat simple.

This follows on from https://github.com/ISISComputingGroup/IBEX/issues/8358

Acceptance criteria

• [ ] CaChannel is modified/inherited to make a PVA wrapper

~Note: make sure we don't end up loading multiple versions of channel access libraries. p4p likely uses epicscorelibs by default, CaChannel uses our own build. May need a custom build of p4p to ensure it uses the same underlying libraries as CaChannel.~ taken care of

Discussed ~1:29 20/06/24

[rerpha]

needed for #8337

[FreddieAkeroyd]

out p4p is already custom built and uses the same epics base libraries as CaChannel, so we shoudl have no conflict

[LowriJenkins]

Genie Python Test Framework Release Notes