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.
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