Allow usage of fixxed IDs
It should be possible to anchor IDs created by an external system.
Currently, we only offer the id_methods "time" and "random".
We could implement this by using only the prefix, and adding an id_method 'none'. With this, the name "prefix" would be misleading.
Otherwise, the method could be called 'static' and we would need to add another key-value pair to input the static ID.
This also is a dependency for a "republishing" workflow (replacing an internal ID with a DOI)