[Task] Support relative references in CoreDocument #1072

Description

Add support for representing references to verification methods and services by fragment only in CoreDocument. In other words the following example from the DID core spec should deserialize to a valid CoreDocument.

Development in this PR

Motivation

Section 3.2.2 of the DID Core spec states that relative references are often used to reference verification methods and services in DID documents.

Resources

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To-do list

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Change checklist

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• The feature or fix is implemented in Rust and across all bindings whereas possible.
• The feature or fix has sufficient testing coverage
• All tests and examples build and run locally as expected
• Every piece of code has been document according to the documentation guidelines.
• If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
• If the feature is not currently documented, a documentation task Issue has been opened to address this.