fdo#72041: Intern strings as we populate the external cache.

This commit covers ods import and UNO API layer.

Change-Id: I3ad1b7cfefe49575694e2351bcba6e5733b009d2
(cherry picked from commit 5706ff70dbb18d03e396fd484bd1392dbcefb6c7)
Reviewed-on: https://gerrit.libreoffice.org/8351
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>