Fix link error using Clang 16.0.5 on Windows

[pmb45]

When using Clang on Windows with Unreal Engine, RiderLink fails to link:

[74/76] Link [x64] UnrealEditor-RiderBlueprint.dll
lld-link: error: undefined symbol: class rd::SynchronousScheduler `public: static class rd::SynchronousScheduler & __cdecl rd::SynchronousScheduler::Instance(void)'::`2'::globalSynchronousScheduler
>>> referenced by C:\Users\...\AppData\Local\Temp\UnrealLink\Pyvexys\HostProject\Plugins\RiderLink\Source\RD\src\rd_framework_cpp\src\main\task\RdCall.h:74
>>>               C:\Users\...\AppData\Local\Temp\UnrealLink\Pyvexys\HostProject\Plugins\RiderLink\Intermediate\Build\Win64\x64\UnrealEditor\Development\RiderBlueprint\Module.RiderBlueprint.cpp.obj:(public: class rd::WiredRdTask<bool, class rd::Polymorphic<bool, void>> __cdecl rd::RdCall<int, bool, class rd::Polymorphic<int, void>, class rd::Polymorphic<bool, void>>::sync(int const &, class std::chrono::duration<__int64, struct std::ratio<1, 1000>>) const)
>>> referenced by C:\Users\...\AppData\Local\Temp\UnrealLink\Pyvexys\HostProject\Plugins\RiderLink\Source\RD\src\rd_framework_cpp\src\main\task\WiredRdTaskImpl.h:65
>>>               C:\Users\...\AppData\Local\Temp\UnrealLink\Pyvexys\HostProject\Plugins\RiderLink\Intermediate\Build\Win64\x64\UnrealEditor\Development\RiderBlueprint\Module.RiderBlueprint.cpp.obj:(public: virtual class rd::IScheduler * __cdecl rd::detail::WiredRdTaskImpl<bool, class rd::Polymorphic<bool, void>>::get_wire_scheduler(void) const)

Moving the static singleton definition to the .cpp file fixes this.

[DecoyRS]

Hey @pmb45 PR looks good, trying to figure out how PR's work right now, because the main branch is now closed for external PR's directly and needs an internal trigger

[DecoyRS]

Looks like I'll need to cherry-pick your changes to a local branch and upload them manually. In short, I approve this PR and merging it. but doing it the hard way. *sigh