Add WhenAnyObservable overloads that combine latest

[kentcb]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature.

What is the current behavior? (You can also link to an open issue here)

The WhenAnyObservable operator does not work in the same fashion as other WhenAny* operators. All observables must be the same type, and the resulting observable sequence is the result of a Merge of those observables. Consider this:

public IObservable<bool> First { ... }

public IObservable<bool> Second { ... }

public IObservable<string> Third { ... }

// elsewhere
this
    .WhenAnyObservable(x => x.First, x => x.Second)
    .Select( /* er, umm, how do I distinguish whether the value came from First or Second? */ )

// somewhere else
this
    .WhenAnyObservable(x => x.First, x => x.Third)  // oops, this is not even possible

What is the new behavior (if this is a feature change)?

This PR adds overloads of WhenAnyObservable that work in the same fashion as other WhenAny* operators. Each observable can be of a different type, and a selector must be provided to combine the results. In our example above, it would look like this:

this
    .WhenAnyObservable(x => x.First, x => x.Second, (first, second) => /* do whatever here to select an appropriate value */)

this
    .WhenAnyObservable(x => x.First, x => x.Third, (first, third) => /* do whatever here to select an appropriate value */)

Does this PR introduce a breaking change?

No.

Please check if the PR fulfills these requirements

• The commit follows our guidelines: https://github.com/reactiveui/reactiveui#contribute
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[ghuntley]

LGTM