Remove apex-pdp unused core modules

Remove apex-pdp unused core modules

This is the final review in the refactoring of the model code in
apex-pdp.

The core-messaging and core-infrastructure modules are unused in
apex-pdp except for websocket communication. Websocket communication is
also not used in any deployment.

This review:
- removes the core-mesaging module, which had an apex-specific websocket
  implementation, except for the MessageException class, which is moved
  to the base core module
- removes the core-infrastructure module, except for the ThreadUtils
  class, which is moved to the base core module
- removes the websocket event protocol, which used core-messaging but is
  not used in any deployment
- collapses all three "core" submodules into a single module
- Updates dependencies where needed

Issue-ID: POLICY-1820
Change-Id: Ieae30063e99570e61943372d3fa23b77a211462c
Signed-off-by: liamfallon <liam.fallon@est.tech>