Record the fact that imported annotations were created via the client's import
mechanism, and the original ID of the annotation. This is currently stored in
the `extra` field. h supports setting arbitrary fields at the top level of
annotations, so we could have put the data there instead. However using the
`extra` field for custom fields is a convention already used by some API
clients, and is distinct from a standard/validated field if we add one later on.

Fixes https://github.com/hypothesis/client/issues/5738