[Task] Support relative references in `CoreDocument`

[olivereanderson]

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.

[olivereanderson]

Here is a list of possible solutions:

1. Make deserialization expand such fragments to full DID URLs by prepending the document's id.
2. Similar to 1. , but also make serialization ignore the DID part of a DID URL (as long as it is the same as the document's id).
3. Expand the MethodRef type to also include a Fragment variant.

Question: Could approach 3. be problematic in the presence of custom JSON-LD @context ?