Deprecation warnings for external scripts using arkindex_cli.commands.export.db

https://redmine.teklia.com/issues/3837

The arkindex_cli.commands.export.db module, which was never intended to be a public API, has been (ab)used by external scripts. We are going to soon rewrite it to use Peewee, and we are not planning to stay compatible with those scripts. We need to add DeprecationWarning warnings and release the CLI with those so that some script users may find out about the issue a little earlier, then we can do the rewrite in the next release.

Those warnings should not be visible when running the CLI itself, otherwise CLI users would be flooded with warnings they cannot do anything about...