Pass --cmdline to func_runner and parse CL args

[csegarragonz]

In this PR I modify the func_runner to optionally take a string to set as command line for the running function. I also take the oportunity to make the argc/argv parsing in func_runner more modular.

In summary, we can now do the following:

# Will print "hello echo"
inv run demo echo --data "hello echo"

# Will print 'hello' and 'world' as arguments
inv run demo argc_argv --cmdline "hello world"

As I am writing this examples, I'm thinking it would be a good idea to have integration tests for the FaasmCLI... 🤔

[Shillaker]

Nice, good to add something more structured for this.

Re. integration tests for Faasm CLI, I think we indirectly test it in the quick-start tests, but not sure if we do it elsewhere. Could be nice if it was a simple extra GHA step.