feat: Add extra attribute to objects

This attribute acts as a namespaced dictionary
and is used by extensions
to add information to the objects.

If a particular projects expects
to find extra data in `obj.extra["project"]`,
then extensions who wish to communicate data
to this project can write into
its "project" namespace/dictionary.

Extensions can also simply write to their own namespace,
using their distribution name for example.

src/griffe/dataclasses.py

@@ -358,6 +358,7 @@ def __init__(
         self.exports: set[str] | list[str | Name] | None = None
         self.aliases: dict[str, Alias] = {}
         self.runtime: bool = runtime
+        self.extra: dict[str, dict[str, Any]] = defaultdict(dict)
         self._lines_collection: LinesCollection | None = lines_collection
         self._modules_collection: ModulesCollection | None = modules_collection