Feature: `Zip` method for fetching an `Outcome`, without losing reference to the original `Outcome`

[sameera]

Scenario:
You need to call two async functions and use results of both the functions in a calculation. Without the ForkJoin, you'll be forced to store the first result on a variable while you call the 2nd function.

A a = null;
await getA()
  .Then(result => {
    a = result;
    return getB();
  })
  .Then(b => Process(a, b));

With forkJoin:

await GetA()
  .ForkJoin(() => GetB()) // optionally .ForkJoin(a => GetB(a)) if that was required.
  .Then((a, b) => Process(a, b));

Signature:

public static Task<Outcome<ValueTuple<A, B>>> ForkJoin<A, B>(
  this Task<Outcome<A>> source, 
  Func<Task<Outcome<B>>> handler
)

[sameera]

Maybe, Zip is a better function name:

await GetA()
  .Zip(() => GetB()) // optionally .Zip(a => GetB(a)) if that was required.
  .Map((a, b) => Process(a, b));

[sameera]

Zip multiple variables?

await GetA()
  .Zip(() => GetB())
  .Zip(() => GetC())
  .Tap((a, b, c) => ... );

// Also
await GetA()
  .Zip(a => GetB(a))
  .Zip((a, b) => GetC(a, b))
  .Tap((a, b, c) => ... );