Python gRPC interceptor that uses Shared Memory

[aaron-ni]

This adds a functioning gRPC interceptor in Python that uses shared memory (SidebandStrategy::SHARED_MEMORY). This required a tweak to the C++ code to add a 4-byte (int32_t) length to the beginning of the sideband_memory space (after the 8 bytes for _writeReady and _readReady).

The top of SharedMemoryForwardingInterceptor.py also includes use_Python_shared_memory that allows toggling between using Python's multiprocessing.shared_memory (which is compatible with what we do) and calling into ni_grpc_sideband.dll directly. The latter is the default and probably preferred because we probably want to move all the memory offset management to C++ for performance and avoiding duplicating details of memory layout that can change over time.

Also, I noticed the C++ code was potentially accessing out-of-bounds memory - InitClientSidebandData and InitOwnerSidebandData were being created at 4096 bytes, but then the first 8 bytes were used for read and write signals and the sideband_memory pointer was incremented by 8. However subsequent memcpy calls still addressed 4096 bytes, meaning they addressed the 8 bytes after the end of what was allocated via Init*SidebandData. In testing this isn't likely to come up, but some memory protections might cause a crash, and of course sometimes prototypes become production code.

[aaron-ni]

FYI @ccifra (I'm not able to add you as a reviewer directly)