Feature: CORE-37263 registry cpp wrapper

[twallis-cpr]

Part of the larger proton re-architecture here (internal CPR/OTTO employee eyes only 👀 )

Part 1 of 3 for proton_core's C++ wrapper.

This PR adds:

• SignalAccess: class for accessing the signal registry in a semi-generic way
• Signal: templated class for abstracting a signal (still requires raw pointer access to the registry)
• SignalBase: non-templated class for signals to live in containers
• RegistryLock: locking primitive for the registry lock for usage with std::lock_guard
• ScopedLock: locking primitive that locks in the constructor, and unlocks in destructor.
• BundleAccess: accessor for bundle API
• PROTON_ENABLE_ALLOC feature flag:
  • Flag enables PC-level features for RTTI (for downcasting SignalBase to Signal<T>)
  • Enables usage of std::function with BundleAccess for lambdas and std::bind for decode callbacks.

[github-actions]

Coverage Summary

Scope	Base Lines	PR Lines	Delta (pp)	Base Functions	PR Functions	Delta (pp)
protonc/protoncpp	lines......: 66.2% (877 of 1325 lines)%	lines......: 66.2% (877 of 1325 lines)%	0.00	functions..: 79.4% (135 of 170 functions)%	functions..: 79.4% (135 of 170 functions)%	0.00
core	lines......: 87.0% (475 of 546 lines)%	lines......: 90.0% (565 of 628 lines)%	0.00	functions..: 84.0% (42 of 50 functions)%	functions..: 99.1% (108 of 109 functions)%	0.00

Coverage deltas are computed as: PR source branch minus target branch (base commit).

Detailed HTML reports are available in the workflow artifacts:

• tests-coverage-report
• core-coverage-report