Add fuzzing target for murxla (#7490)
This PR aims to make it easier for cvc5 developers to fuzz the current cvc5 working copy with murxla.
It adds a new external project Murxla-EP that builds murxla, linking (only) against cvc5. It depends (indirectly) on the make install target, which is called with a custom install prefix for this purpose.
As the murxla repository is not public yet, automatically downloading it does not work, but instead we need the user to clone it manually for now.
The user should simply execute make fuzz-murxla and follow the instructions.