Handle the different data (string, URL) that may be provided as configured DOI or CodeMeta identifier

[sdruskat]

CodeMeta identifier values can be either a URL or a string (propertyValue). Similarly, users may configure a DOI resolver URL rather than a pure DOI as deposit.invenio.doi.

We should

1. safeguard against these cases by checking the data before trying to resolve DOIs in deposit. (Perhaps in a util module, as this is likely to be an issue we have to handle in other contexts as well).
2. clearly document the valid values for the deposit.invenio.doi configuration field.