parent-doi: add configuration

[utnapischtim]

should fix https://github.com/inveniosoftware/invenio-rdm-records/issues/1652

QUESTIONS:

• is this a valid approach?
• should the default be:
  • yes create parent dois
  • no parent doi should be created
• the PR implements it as DATACITE_ENABLE, where it has to be explicitly turned on the problem with that is, that it is not backwards compatible to the actual code, BUT it would be backwards compatible to v11
• i added a check to the migration script, to only create parent DOI's if datacite is enabled (PR). if the PARENT_DOI_ENABLE config will be merged this should be added to the migration script too