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
the gateway was supposing that comments where emitted from PEP of author. While this is
the case for most blog posts, it's not for comments. Instead the component is now using
`author_jid` which is retrieved by XEP-0277 plugin, and reject the item if the auhor is
not verified (i.e. if `publisher` attribute is not set by XMPP service).
