fix: add Linked VPs to IOTA DID document

[daniel-mader]

Description of change

Ensure IOTA DID document is updated when Linked VP is added.

Links to any relevant issues

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How the change has been tested

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Definition of Done checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
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• I have successfully tested this change in a docker environment