[Task] Improve Account::attach/detach_method_relationship

[cycraig]

Description

Modify Account::attach_method_relationship and Account::detach_method_relationship:

• pluralise the names to match the Wasm bindings Account, i.e. attach_method_relationships and detach_method_relationships.
• allow specifying multiple relationships at once rather than individually, this would allow passing collections of relationships.

Current behaviour:

account
  .update_identity()
  .attach_method_relationship()
  .fragment("key1")
  .relationship(MethodRelationship::Authentication)
  .relationship(MethodRelationship::CapabilityInvocation)
  .apply()
  .await?;

Desired behaviour:

account
  .update_identity()
  .attach_method_relationships()
  .fragment("key1")
  .relationships(&[MethodRelationship::Authentication, MethodRelationship::CapabilityInvocation])
  .apply()
  .await?;

The .relationships implementation can either take a slice, vec, or arbitrary IntoIter generic depending on which we find is most convenient for developers to use.

We should also think about making attaching/detaching relationships easier for developers, since it is currently restricted to non-embedded methods. That is, trying to attach/detach an embedded method always causes an error, we could instead move the method to be a verificationMethod.

Motivation

Porting the Account for the Wasm bindings in #574 demonstrated that using the builder pattern to add/remove multiple method relationships is cumbersome due to ownership/borrowing rules on intermediate structs. This issue is an attempt to make these specific methods more user-friendly and to reduce differences with the Wasm bindings.

E.g. current code needed to add relationships from a list, which is definitely not ergonomic:

// The list is static here but is usually defined programmatically.
let relationships: Vec<MethodRelationship> = vec![
  MethodRelationship::Authentication, 
  MethodRelationship::CapabilityInvocation,
];
// Combining any of these lines angers the borrow checker.
let mut account: RefMut<Account> = account.borrow_mut();
let mut updater: IdentityUpdater<'_> = account.update_identity();
let mut attach_relationship: AttachMethodRelationshipBuilder<'_> =
  updater.attach_method_relationship().fragment(fragment);

for relationship in relationships {
  attach_relationship = attach_relationship.relationship(relationship);
}

attach_relationship.apply().await?;

To-do list

• rename Account::attach_method_relationship to attach_method_relationships
• rename Account::detach_method_relationship to detach_method_relationships
• allow Account::attach_method_relationships to specify multiple relationships at once.
• allow Account::detach_method_relationships to specify multiple relationships at once.

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.