Expose iteration over verification relationship fields

[abdulmth]

Description of change

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

Links to any relevant issues

fixes issue #989

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

[PhilippGackstatter]

Looks good to me!

I tried a different approach with Box<dyn Iterator> but it produces a 6 KiB larger Wasm binary, so the Vec approach seems preferable despite more allocations and going through the entire iterator upfront.

For reference:

pub fn methods<'this>(
    &'this self,
    scope: Option<MethodScope>,
  ) -> Box<dyn Iterator<Item = &VerificationMethod<D, U>> + 'this>
  where
    D: DID,
  {
    if let Some(scope) = scope {
      match scope {
        MethodScope::VerificationMethod => Box::new(self.verification_method().iter()),
        MethodScope::VerificationRelationship(MethodRelationship::AssertionMethod) => Box::new(
          self
            .assertion_method()
            .iter()
            .filter_map(|mr| self.resolve_method_ref(mr)),
        ),
        MethodScope::VerificationRelationship(MethodRelationship::Authentication) => Box::new(
          self
            .authentication()
            .iter()
            .filter_map(|mr| self.resolve_method_ref(mr)),
        ),
        MethodScope::VerificationRelationship(MethodRelationship::CapabilityDelegation) => Box::new(
          self
            .capability_delegation()
            .iter()
            .filter_map(|mr| self.resolve_method_ref(mr)),
        ),
        MethodScope::VerificationRelationship(MethodRelationship::CapabilityInvocation) => Box::new(
          self
            .capability_invocation()
            .iter()
            .filter_map(|mr| self.resolve_method_ref(mr)),
        ),
        MethodScope::VerificationRelationship(MethodRelationship::KeyAgreement) => {
          Box::new(self.key_agreement().iter().filter_map(|mr| self.resolve_method_ref(mr)))
        }
      }
    } else {
      Box::new(self.all_methods())
    }
  }

[cycraig]

Looks good to me.