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>

ipalib/frontend.py

@@ -29,6 +29,7 @@ from ipapython.ipautil import APIVersion
 from ipalib.base import NameSpace
 from ipalib.plugable import Plugin, APINameSpace
 from ipalib.parameters import create_param, Param, Str, Flag
+from ipalib.parameters import create_signature
 from ipalib.parameters import Password  # pylint: disable=unused-import
 from ipalib.output import Output, Entry, ListOfEntries
 from ipalib.text import _
@@ -37,7 +38,7 @@ from ipalib.errors import (ZeroArgumentError, MaxArgumentError, OverlapError,
     ValidationError, ConversionError)
 from ipalib import errors, messages
 from ipalib.request import context, context_frame
-from ipalib.util import classproperty, json_serialize
+from ipalib.util import classproperty, classobjectproperty, json_serialize
 
 if six.PY3:
     unicode = str
@@ -433,6 +434,26 @@ class Command(HasParam):
 
     topic = classproperty(__topic_getter)
 
+    @classobjectproperty
+    @classmethod
+    def __signature__(cls, obj):
+        # signature is cached on the class object
+        if hasattr(cls, "_signature"):
+            return cls._signature
+        # can only create signature for 'final' classes
+        # help(api.Command.user_show) breaks because pydoc inspects parent
+        # classes and baseuser plugin is not a registered object.
+        if cls.__subclasses__():
+            cls._signature = None
+            return None
+        # special, rare case: user calls help() on a plugin class instead of
+        # an instance
+        if obj is None:
+            from ipalib import api
+            obj = cls(api=api)
+        cls._signature = signature = create_signature(obj)
+        return signature
+
     @property
     def forwarded_name(self):
         return self.full_name