Refactor scriptlet loader and improve checks

[bensmrs]

The error generation function is pretty heavy, but I think the error messages are way nicer this way.

root@test-incus:~# incus config set authorization.scriptlet - <<EOF
def get_project_access(project):
  return ['ben']

get_instance_access = 3
EOF
Error: cannot set 'authorization.scriptlet' to 'def get_project_access(project):
  return ['ben']

get_instance_access = 3
': the function "authorize" is required but has not been found in the scriptlet; additionally, "get_instance_access" should define the scriptlet’s optional function of the same name (found a value of type int instead); finally, the optional function "get_project_access" defines arguments ["project"] (expected ["project_name"])

[stgraber]

The optional function handling will be quite convenient :)