[Task] Expose iteration over verification relationship fields

[cycraig]

Description

Add an optional scope parameter to the method getter on DID Document implementations to allow iterating over particular verification relationship fields.

E.g.

/// Returns an iterator over all [`<DID>VerificationMethods`] in the DID Document.
pub fn methods(&self, scope: Option<MethodScope>) -> impl Iterator<Item = &<DID>VerificationMethod> {
  ...
}

Edit: alternatively we could just delegate the individual authentication(), assertionMethod(), etc. functions.

Motivation

Currently, there is no way to list or iterate over the methods with a particular verification relationship (authentication, assertionMethod, keyAgreement, etc.) in IotaDocument and StardustDocument other than accessing the inner CoreDocument which has dedicated methods for each (authentication(), assertionMethod(), keyAgreement(), etc,).

It would be convenient to expose this functionality without bloating the API with methods for each field, similar to how resolve_method works now.

To-do list

• Add optional scope parameter to methods().

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.