[WIP] Settings validation

[jcardonnet]

Fixes #1664, #1659

[khoaguin]

Maybe this is also related to this ticket https://github.com/OpenMined/Heartbeat/issues/1649, since after updating the settings with

client.api.services.settings.update(
    name="a",
    organization="b",
    description="c",
    admin_email="[email protected]",
    association_request_auto_approval=True,
    eager_execution_enabled=False,
    notifications_enabled=True,
)
client.refresh()

doing guest_client.register will produce the below error