[Caspar] Python bindings opt-out

[tordnat]

Projects embedding only the CXX/CUDA core of Caspar (e.g. via ExternalProject_Add) can now skip pybind11 entirely:

• CasparLibrary.generate(..., python_bindings=False) skips generating pybinding.cc, caspar_mappings_pybinding.h, solver_pybinding.h, and .pyi stubs
• The generated CMakeLists.txt omits the pybind11 find/fetch block and pybind11_add_module target entirely when python_bindings=False
• When python_bindings=True (default), a CASPAR_BUILD_PYTHON_BINDINGS CMake option is still emitted for compile-time opt-out
• All existing callers are unaffected since the default is True

Blocked by #457 and requested by colmap/colmap#4018 such that we don't have to manually remove python bindings when regenerating.

[tordnat]

New changes tested on bal example. Runs fine, but blocked by #458 since the pybinding fixes have not been applied to this branch yet. Added a new filter for linking which should avoid generating pybind headers as well:

if not python_bindings and "pybind" in f.name:
    continue