We currently have no usage of host_connector/byte_buf.h. Should we be using it? Can we delete it?

There are not many instances in the existing code of transferring ownership of a ByteBuf from the extension to host. One instance is here though, where we create a new VecByteBufHandle by static casting the passed buffer to VecByteBuf. This is a danger of using shared code between the host connector and SDK: if the extension was written in Rust, the byte buf would not be a C++ VecByteBuf and this static cast would not work. Maybe we should consider using ExtensionByteBufHandle here so that we can take ownership of the buffer while only using the opaque type ::MongoExtensionByteBuf.