Make QCryptographicHash providers a plugin

This can let us more easily provide more backends as well as keep the original 'Qt version' separate.
For example, we could provide OS-specific backends that does not need OpenSSL to be linked dynamically. This reduces friction and let's us enable this secondary backend in our shipped releases.

Whether we go with compile-time selection of files to compile into Core or run-time Qt plugin loading is up for decision. But there might not be a need to switch backends at runtime.

For export restriction reasons the maintainer of the module cannot currently touch the OpenSSL-using code, which makes (re-)work, and likely future bug-fixing, harder.