[Feature]: New Operator GroupOnObservable

[dwcullop]

As suggested in #409, we should consider something like GroupOnObservable with a prototype such as:

 IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> GroupOnObservable<TObject, TKey, TGroupKey>(
    this IObservable<IChangeSet<TObject, TKey>> source,
    Func<TObject, TKey, IObservable<TGroupKey>> groupOnObservable);

Which would group the values based on the last value returned from each sub-observable. When a new value is fired, the corresponding value would move from one group to another.

I was considering something like GroupAsync but this operator would cover that use-case (and a lot more).

Originally posted by @dwcullop in #672 (comment)

[dwcullop]

Implemented by #847.

[github-actions]

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