Ticket 50633 - Add cargo vendor support for offline builds
    
    Bug Description: At suse/rh we need to be able to build offline. To
    achieve this we need offline builds. This adds support for these in
    389-ds with cargo and rust.
    
    Fix Description:
    This adds cargo vendor support for offline builds,
    and shows that they work. We add a stub library for librslapd/libslapd
    so that we can begin to develop features in rust.
    
    To build normally: work as usual.
    
    To build offline: make -f rpm.mk download-cargo-dependencies
        ./configure --enable-rust --enable-rust-offline
    
    Continue to build as usual.
    
    A note to keep in mind is cargo test does not work offline as
    dev-dependencies are not vendored.
    
    The download-cargo-dependencies has been added to dist-bz2 for
    distributions.
    
    https://pagure.io/389-ds-base/pull-request/50633
    
    Author: William Brown <william@blackhats.net.au>
    
    Review by: mhonek (Thanks)