Add __signature__ to plugins
    
    Auto-generate inspect.Signature from plugin arguments and options. The
    signature is used by (amongst others) pydoc / help.
    
    ```
    $ ipa console
    >>> help(api.Command.group_add)
    Help on group_add in module ipaserver.plugins.group object:
    
    class group_add(ipaserver.plugins.baseldap.LDAPCreate)
     |  group_add(cn: str, *, description: str = None, gidnumber: int = None, setattr: List[str] = None, addattr: List[str] = None, nonposix: bool, external: bool, all: bool, raw: bool, version: str = None, no_members: bool) -> Dict[str, Any]
    ```
    
    Fixes: https://pagure.io/freeipa/issue/8388
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>