identifiers: added related identifiers config var

[0einstein0]

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Description

In relation to: CERNDocumentServer/cds-rdm#574

Checklist

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• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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