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 #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