Update ServiceEndpoint to support sets and maps

[cycraig]

Description of change

Update the serviceEndpoint field of Service to allow an ordered set or map as well as the current single URL. Note that "ordered" refers to maintaining the order of insertion, not comparison.

/// An endpoint or set of endpoints specified in a [`Service`].
///
/// [Specification](https://www.w3.org/TR/did-core/#dfn-serviceendpoint)
#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
#[serde(untagged)]
pub enum ServiceEndpoint {
  One(Url),
  Set(OrderedSet<Url>),
  Map(IndexMap<String, OrderedSet<Url>>),
  // TODO: enforce set/map is non-empty?
}

This is in line with the DID specification: https://www.w3.org/TR/did-core/#dfn-serviceendpoint

The value of the serviceEndpoint property MUST be a string, a map, or a set composed of one or more strings and/or maps. All string values MUST be valid URIs conforming to [RFC3986] and normalized according to the Normalization and Comparison rules in RFC3986 and to any normalization rules in its applicable URI scheme specification.

One thing to note is that this PR allows empty sets and maps while the spec specifies "one or more". This is mostly due to ergonomics to avoid clunky wrapping types but can be changed if necessary.

Also according to the spec, we should allow nested maps in serviceEndpoint sets and maps too. Considering that would complicate the Rust types and usability quite a bit, it was decided not to support nested maps at this time.

Links to any relevant issues

Resolves issue #447.

Type of change

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• 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

How the change has been tested

Describe the tests that you ran to verify your changes.
Make sure to provide instructions for the maintainer as well as any relevant configurations.

Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[olivereanderson]

Good work. Appreciate the new tests to the changed/new functionality.