[broker] updated relation descriptors #127

This PR aims to align the procedure for creating the broker events with the updated relation descriptors introduced in dhp-schemas:2.7.14.

I'm not 100% familiar with the implementation, for the time being I updated what I noticed was surely a reference to the relation labels that needed to be aligned with the new speficication, introducing

The usage of constant values from ModelConstants;
Updating the recognition of the dedup root ids.

The full list of changes is available at https://code-repo.d4science.org/D-Net/dhp-schemas/src/tag/dhp-schemas-2.7.14/CHANGES.md

Please verify if we need more changes.

This PR aims to align the procedure for creating the broker events with the updated relation descriptors introduced in `dhp-schemas:2.7.14`. I'm not 100% familiar with the implementation, for the time being I updated what I noticed was surely a reference to the relation labels that needed to be aligned with the new speficication, introducing * The usage of constant values from `ModelConstants`; * Updating the recognition of the dedup root ids. The full list of changes is available at https://code-repo.d4science.org/D-Net/dhp-schemas/src/tag/dhp-schemas-2.7.14/CHANGES.md Please verify if we need more changes.

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[broker] updated relation descriptors #127

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