[Feature]: Papers should have several external IDs

[Slartibartfass2]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe

Currently, we only store a single external ID, but there can be more from different provider.

Describe the solution you'd like

Replace the single external ID with a list of them as discussed in SE-UUlm/snowballr-backend#514.

Describe alternatives you've considered

No response

Anything else?

No response