suricata: expose the SuricataContext with a function

suricata: expose the SuricataContext with a function

Expose the "SuricataContext" required by Rust as a function. During
normal startup we register this context with the Rust code, but
plugins written in Rust will need to get the same registration
done, but to do this in a plugin, the plugin code must
call and set the context within its address space.