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

"""${message}

Revision ID: ${up_revision}
Revises: ${down_revision | comma,n}
Create Date: ${create_date}

"""
from alembic import op
import sqlalchemy as sa
${imports if imports else ""}

# revision identifiers, used by Alembic.
revision = ${repr(up_revision)}
down_revision = ${repr(down_revision)}
branch_labels = ${repr(branch_labels)}
depends_on = ${repr(depends_on)}


def upgrade():
    ${upgrades if upgrades else "pass"}


def downgrade():
    ${downgrades if downgrades else "pass"}