[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.

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

Create a task-specific to-do list. Please link PRs that match the TODO list item behind the item after it has been submitted.

• [ ] Item one
• [ ] Item two
• [ ] Item three

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 ?