[Task] Enforce intra-document fragment uniqueness

[eike-hass]

Description

We should enforce uniqueness of fragments in DID documents, since multiple identical fragments in one document could lead to errors or at least unexpected or undefined behavior, during credential validation and other operations.

Motivation

Prevent user errors and keep documents in a consistent state.

Sub-tasks

• [ ] Ensure no two methods in the document have the same identifier
• [ ] Ensure services and methods have different identifiers
• [ ] Ensure custom properties have unique identifiers.

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• [ ] 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.