Add support for relating to DICOM fields directly in the schema

[yarikoptic]

Inspired by

• [ENH] Add explicit wording on DICOM terms correspondence #1450 (comment)

ATM we have not schematized the relationships we might have between our metadata fields and (meta)data from DICOMS. We express it in free word form and that is why we lack consistency (hence #1450 and fresh #1758). I think, with possible future extensions in mind, we should add to schema for metadata fields

relations:
   dicom:
      {relation}:
         - TAG

e.g.

relations:
  dicom:
    identicalTo:
      - 0002, 0003
      - 0002, 0008

or

relations:
  dicom:
    derivedFrom:
      - 0002, 0003
      - 0002, 0008

where

• those relations are part of https://support.datacite.org/docs/connecting-to-works#summary-of-all-relationtypes without is
• can't have both identicalTo and derivedFrom (until we run into use case).

This would

• allow for consistent rendering and annotation of relations
• allow to hyperlink to DICOM standard description for those fields
• avoid necessity for verbal description in common principles such as proposed in [ENH] Add explicit wording on DICOM terms correspondence #1450 (comment)
• make it generic, possibly opening possibility to relate to nidm terms (attn @dbkeator , Repository of BIDS terms and supporting the BIDS community #423)
• allow for verification of validity of values by the "schema validator (tests)"