Feature: EditDiff extension method for IObservable<IEnumerable<T>>

[dwcullop]

Description

When #736 completes, it will be handy to have more ways to create child changesets that can be merged together and treated as a single stream of changes.

To that end, the PR adds an EditDiff extension method to IObservable<IEnumerable<T>> to create a changeset from a collection that changes over time. It is similar to (and internally leverages) the EditDiff extension method for ISourceCache that updates the contents of a cache to match a given collection by only applying the changes. However, it operates on Observable collections and updates a dedicated cache each time a new collection is observed. It is the counterpart to ToCollection.

This method is distinct from ToObservableChangeSet that only adds/updates each item in each observed collection (with an option to expire). EditDiff will remove values if they're NOT observed in subsequent versions of the collection.

Example

interface IPerson : INotifyPropertyChange
{
  int Id { get; }
  string Name { get; }
  Optional<IPerson> Spouse { get; }
  IReadOnlyList<IPerson> Children { get; }
}

IObservableCache<IPerson, int> peopleCache = GetPeopleCache();

// Treat all changes to all children collections as one observable changeset
IObservable<IChangeSet<IPerson, int>> childrenChangeSet = 
    peopleCache.Connect()
               .MergeManyChangeSets(person => person.WhenAnyValue(p => p.Children).EditDiff(p => p.Id));

Performance

I've included a Perf test in the unit tests. The performance is reasonable, but scales with the size of the collections, and I am open to any ideas around improving performance. However, since it leverages the single collection version of EditDiff, it would be more appropriate to put any optimizations in there (but it seems fairly optimized already).

[codecov]

Codecov Report

Merging #738 (dfab716) into main (f1da1b1) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
+ Coverage   72.22%   72.25%   +0.03%     
==========================================
  Files         217      218       +1     
  Lines       11057    11070      +13     
==========================================
+ Hits         7986     7999      +13     
  Misses       3071     3071              

Files	Coverage Δ
...rc/DynamicData/Cache/Internal/EditDiffChangeSet.cs	100.00% <100.00%> (ø)
src/DynamicData/Cache/ObservableCacheEx.cs	51.74% <100.00%> (+0.19%)	⬆️

[github-actions]

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