Introduce --prover-option flag for CBMC

[rod-chapman]

I propose the main "cbmc" command should introduce a new

--prover-option=XXX

command-line option. If specified, then "XXX" is passed to the underlying prover (whichever one it is) to offer a little more control of its behaviour.

For example

cbmc --smt2 --prover-option="tactic.default_tactic=smt" mything.goto

would invoke

z3 tactic.default_tactic=smt ...

under the hood, and similarly for other provers like bitwuzla, cadical, etc. etc.