Unify APEX tracing backend under hpx::tracing API

[v4xsh]

Proposed Changes

• Integrate the APEX tracing backend into the unified hpx::tracing layer, replacing scattered HPX_HAVE_APEX handling across runtime call sites.
• Wire the tracing module into the core futures and runtime_local targets, and clean up the Tracy setup path so tracing target usage stays consistent.
• Break the tracing / threading_base dependency cycle by keeping the APEX-specific timer implementation safely within external_timer.cpp.
• Migrate key execution paths to the unified tracing API, including thread scheduling, task execution, runtime startup/finalization, parcel send/receive handling, distributed action posting, and performance counter sampling.
• Replace older APEX annotation plumbing with task_timer_data and no-op fallback paths so tracing stays strictly lightweight when the backend is disabled.

Any background context you want to provide?

This PR completes the APEX side of the tracing unification by moving APEX runtime instrumentation behind the common hpx::tracing interface. Before this, APEX-specific hooks and HPX_HAVE_APEX conditionals were heavily spread across threading, parcelset, actions, counters, and runtime initialization code.

With this change, those call sites use the shared tracing API instead, while the APEX-specific implementation is kept delegated to the threading_base external timer implementation. This makes the instrumentation significantly easier to maintain, reduces direct coupling between modules, and keeps disabled tracing paths as lightweight zero-overhead fallbacks.

Checklist

Not all points below apply to all pull requests.

• I have added a new feature and have added tests to go along with it.
• I have fixed a bug and have added a regression test.
• I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

[StellarBot]

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[codacy-production]

Up to standards ✅

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[hkaiser]

@v4xsh inspect reports:

/libs/core/threading_base/src/external_timer.cpp: *I* missing #include (string) for symbol std::string on line 155

[hkaiser]

Can we make these functions 100% no-ops if APEX is disabled?

[hkaiser]

I think we're at the point where we can separate the code for the various backends into separate header files.

[v4xsh]

@hkaiser Kindly have a look!