Add script for populating attachments

[joelvdavies]

Useful for development and testing. E.g. the db-import in https://github.com/ral-facilities/inventory-management-system-api/blob/develop/scripts/dev_cli.py.

While we could use the same method of creating a dump for preserving the MongoDB data, I don't believe it will be possible for MinIO. It will also need to link to entities on IMS, so we will need to obtain IDs from there to add them here and using dumps would not work as well as a result. Ideally future download requests would fail gracefully if the object was never uploaded, so arguably we may be able to avoid having actual files for each attachment created with the script.

[joelvdavies]

Probably worth having an option in the script to generate mock ones for a specific entity id so can be more targeted. Added ral-facilities/inventory-management-system-api#392 as a pre-requisite as I think it could be useful to reduce the total number anyway.