Make common CMake targets more modular

[saki7]

During transition of my non-iris project, I found that ASan related flags are not quite usable in certain scenario. In game projects, we have to link our library to the game engine SDK that is built with some preconfigured flags, which makes it often not that easy to "just enable ASan everywhere".

Ideally, we should split test-related common targets (i.e. iris_cxx_test iris_cxx_test_external) into two variations: sanitizer-enabled version and disabled version. However, that requires a lot of work to implement because the normal CMake dependencies added by FetchContent (e.g. Catch2) cannot be duplicated, because their targets are defined inside their own hard-coded CMake namespace (e.g. Catch2::Catch2).

In order to do the complete fix, I think we need to use ExternalProject_Add (instead of FetchContent) and build two versions of all dependent libraries. That should be addressed in a future PR.

This PR adds a new option IRIS_TEST_USE_SANITIZER (defaulted to ON). Users may turn it OFF to globally disable sanitizer in non-sanitizer-applicable projects.

[cppwarningnotifier]

Environment				C++23	C++26
iris	Clang	21	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
	MSVC	2022	Debug	✅success	✅success
			Release	✅success	✅success
		2026	Debug	✅success	✅success
			Release	✅success	✅success