core: get rid of `pickle`: Use of `pickle` to serialise data was a technical legacy that was causing trouble to store in database, to update (if a class was serialised, a change could break update), and to security (pickle can lead to code execution). This patch remove all use of Pickle in favour in JSON, notably: - for caching data, a Pydantic model is now used instead - for SQLAlchemy model, the LegacyPickle is replaced by JSON serialisation - in XEP-0373 a class `PublicKeyMetadata` was serialised. New method `from_dict` and `to_dict` method have been implemented to do serialisation. - new methods to (de)serialise data can now be specified with Identity data types. It is notably used to (de)serialise `path` of avatars. A migration script has been created to convert data (for upgrade or downgrade), with special care for XEP-0373 case. Depending of size of database, this migration script can be long to run. rel 443

--- a/libervia/backend/plugins/plugin_comp_ap_gateway/http_server.py	Fri Feb 16 18:46:06 2024 +0100
+++ b/libervia/backend/plugins/plugin_comp_ap_gateway/http_server.py	Fri Feb 23 13:31:04 2024 +0100
@@ -1017,7 +1017,7 @@
             if len(extra_args) != 1:
                 raise exceptions.DataError("avatar argument expected in URL")
             avatar_filename = extra_args[0]
-            avatar_path = self.apg.host.common_cache.getPath(avatar_filename)
+            avatar_path = self.apg.host.common_cache.get_path(avatar_filename)
             return static.File(str(avatar_path)).render(request)
         elif request_type == "item":
             ret_data = await self.apg.ap_get_local_object(ap_url)