Add support for awaiting async functions from Python port

[josead]

Add support for awaiting async functions from Python port:

• Implement py_loader_port_await in py_loader_port.c with resolve/reject callbacks that bridge MetaCall's async mechanism to Python Futures
• Add accessor functions for asyncio_loop and thread_background_module in py_loader_impl.c/.h
• Add metacall_await function and MetaCallFunction class to api.py supporting both sync and async function calls
• Update find_handle to use MetaCallFunction with automatic async detection via metacall_inspect metadata
• Export new symbols in init.py
• Add comprehensive Python unit tests in test_await.py (~35 tests)
• Add C++ integration tests in metacall_python_port_await_test
• Update CMakeLists.txt to include new test targets
• Fix missing 'import threading' in metacall_python_await_test.cpp

Description

Please include a summary of the change and which issue is fixed. List any dependencies that are required for this change.

Fixes #(issue_no)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Documentation update

Checklist:

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests/screenshots (if any) that prove my fix is effective or that my feature works.
• I have tested the tests implicated (if any) by my own code and they pass (make test or ctest -VV -R <test-name>).
• If my change is significant or breaking, I have passed all tests with ./docker-compose.sh test &> output and attached the output.
• I have tested my code with OPTION_BUILD_ADDRESS_SANITIZER or ./docker-compose.sh test-address-sanitizer &> output and OPTION_TEST_MEMORYCHECK.
• I have tested my code with OPTION_BUILD_THREAD_SANITIZER or ./docker-compose.sh test-thread-sanitizer &> output.
• I have tested with Helgrind in case my code works with threading.
• I have run make clang-format in order to format my code and my code follows the style guidelines.

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