chore: extract all TODOs/FIXMEs to issues

[slint]

The code base of this module has already 14 of these and growing 😓

_Originally posted by @ppanero in https://github.com/inveniosoftware/invenio-rdm-migrator/pull/119#discussion_r1280201623_

[ppanero]

issues:

• https://github.com/inveniosoftware/invenio-rdm-migrator/issues/124
• https://github.com/inveniosoftware/invenio-rdm-migrator/issues/123
• https://github.com/inveniosoftware/invenio-rdm-migrator/issues/122
• https://github.com/inveniosoftware/invenio-rdm-migrator/issues/125