[10.x] Standardize withCount() & withExists() eager loading aggregates.

[r-kujawa]

🔴 There has been an attempt to add this functionality in #41914 but got reverted in #41943 due to a bug in the code. I figured I'd modify the function signatures to match the with() function and send it to 10.x instead.

What does this PR do?

• Standardizes the way you can eager load your withCount() & withExists() aggregates.
• Adds logic to existing tests to support this functionality☝🏼.

Why do we need this?

Before this PR

There are 3 different ways to pass parameters in with...() functions:

1. Passing relation strings (internally will check is_array($relations) ? $relations : func_get_args()).
2. Passing an array of relations (with or without a closure as the value).
3. Passing the first parameter as a relation string and second argument as a closure.

If we compare the with(), withCount() & withExists functions, this is what each of them currently support:

• with(): 1, 2 & 3.
• withCount(): 1 & 2.
• withExists(): 1 (with only 1 relation 😞) & 2.

The reason why I am writing this PR is because I ran into a situation where I assumed that since I was able to do it with one of these methods, why shouldn't I be able to do the same with any of the other 2 methods.

With this PR

The 3 functions mentioned above will support all 3 different ways to pass the params.

[taylorotwell]

I don't see a reason to introduce a breaking change the community isn't widely asking for.