[Task] Support relative references in `CoreDocument`

## 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](https://www.w3.org/TR/did-core/#example-an-example-of-a-relative-did-url) should deserialize to a valid `CoreDocument`.

Development in this [PR](https://github.com/iotaledger/identity/pull/1341)

## Motivation
Section [3.2.2 of the DID Core spec](https://www.w3.org/TR/did-core/#relative-did-urls) 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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- [ ] 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.