feat(ntp): eternal singleton for NtpTimeService with optional DLL linkage

[NewYaroslav]

Motivation

• Replace the previous ad-hoc singleton storage with a robust "eternal" singleton pattern that supports both header-only and DLL-backed singletons.
• Allow a single process-wide instance when building as a DLL to avoid ODR/multiple-instance issues.
• Keep changes minimal and localized to ntp_time_service.hpp and tests.
• Add an automated check that the DLL-singleton path compiles/links correctly under ODR test scenarios.

Description

• Introduced a detail::NtpTimeServiceSingleton template and switched NtpTimeServiceT::instance() to return detail::NtpTimeServiceSingleton<RunnerT>::instance().
• Added optional DLL macros: TIME_SHIELD_NTP_TIME_SERVICE_USE_DLL_SINGLETON, TIME_SHIELD_NTP_TIME_SERVICE_DLL_EXPORTS, and TIME_SHIELD_NTP_TIME_SERVICE_DLL_IMPLEMENTATION and TIME_SHIELD_NTP_TIME_SERVICE_API to control exported ntp_time_service_instance() linkage.
• Implemented the DLL-exported ntp_time_service_instance() (when DLL_IMPLEMENTATION is defined) and kept the header-only eternal-allocated instance as the default.
• Updated tests: removed the old define from tests/ntp_time_service_test.cpp, added ODR test sources tests/odr/ntp_time_service_dll_a.cpp and tests/odr/ntp_time_service_dll_b.cpp, and wired them into tests/odr/CMakeLists.txt to validate the DLL-singleton code path.

Testing

• Configured tests and generated build files with cmake -S . -B build -DTIME_SHIELD_CPP_BUILD_TESTS=ON and configure succeeded.
• Built the project with cmake --build build and the build completed successfully.
• Executed tests with ctest --test-dir build and all tests passed (36/36).
• The new ODR/DLL test target ntp_time_service_dll_odr was built and ran as part of the test suite.

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