feat(combineLatest): add N-args signature for observable inputs

[ggradnig]

Description:
Adds a type signature to provide an array with more than six elements to combineLatest without losing the order of the input observable types. We recently had this problem at a client project and needed to resort to nesting combineLatest.

The solution makes use of TypeScript readonly tuples. They allow for an array [a, b] to have the type [A, B] instead of (A | B)[]. With this type, we can make use of ObservedValueTupleFromArray to get all the observed types of the tuple in the correct order.

This comes with the "drawback" that the input array must be declared as const, else TypeScript assumes a union type per default. It would work with spreading syntax though like in combineLatestWith, but this signature was deprecated for combineLatest.

For this to work, the conditional type ObservedValueTupleFromArray must check for the narrow readonly T[] type, but this shouldn't be an issue as T[] extends it.

Related issue:
#5066

observable/combineLatest.ts - needs N-args signature for observable inputs? See also #5209

#4410

[rraziel]

@ggradnig this one could be useful to you as well: #5363

[ggradnig]

Yeah thanks for the hint, but this pretty much already exists in https://github.com/cartant/rxjs-etc
The reason I can't use that here is that I want to spread the result array directly into another function. Can't do that easily with objects...
This PR just adds a type signature without any runtime changes to the existing combineLatest

[benlesh]

Thank you @ggradnig!