Reject any non-encrypted data sent by a client

While we encrypt everything client-side in Brave, Chromium supports non-encrypted Sync as well. We should therefore add a server-side check to detect any client bugs that end up sending unencrypted data to the backend.

This could simply involve checking this field: https://github.com/brave/go-sync/blob/e2356a7a31a88d7f6707ae0dbfd18a001e970ed4/schema/protobuf/sync_pb/entity_specifics.proto#L76

Start with the referenced entity_specifics.proto field and trace where the server accepts client entity data. Determine how that field indicates encryption, then add server-side rejection for non-encrypted data and verify that encrypted client data remains accepted. Done means unencrypted client payloads are rejected consistently.

Written by the indexing model from the issue text.