Added new method to StateContainer that takes a StateSequence instace

Description

I've been using VSM more and more inside of Swift Concurrency code and notice that I keep repeating the same mistake over and over. When ever I write a function on a state model that returns a StateSequence, I mark that function as async even though inside said function I don't call any async functions. I believe it's because current the only way to have StateContainer to observe a StateSequence you need to call the observeAsync method which takes an async closure that returns a StateSequence.

Take this method as an example;

func getFirstPage(using datasource: Datasource) async -> StateSequence<PagedListViewState> {
    .init({ .fetchingFirstPage }, { await fetchingPage(using: datasource) })
}

I wrote it this way because when I call it I typically write it like this:

$state.observeAsync { await viewModel.getFirstPage(using: datasource) }

But this is entirely unnecessary because creating a new instance of StateSequence is not an async options. StateSequence models an array of async closures that return a State. iterating through the sequence is an async operation.

So in this PR I added a new overload to the observe method that I think is a better fit when you need to only observer a StateSequence, but creating that sequence is not asynchronous. Taking the same code as above I would now write the function that returns the StateSequence like this:

func getFirstPage(using datasource: Datasource) -> StateSequence<PagedListViewState> {
    .init({ .fetchingFirstPage }, { await fetchingPage(using: datasource) })
}

And when I go to observe those state changes I would call the new observe method that would look like this:

$state.observe(viewModel.getFirstPage(using: datasource))

I think this should prevent other users from making the same mistake I keep making.

Type of Change

[ ] Bug Fix
[x] New Feature
[ ] Breaking Change
[ ] Refactor
[ ] Documentation
[ ] Other (please describe)

Checklist

[x] I have read the contributing guidelines
[x] Existing issues have been referenced (where applicable)
[x] I have verified this change is not present in other open pull requests
[x] Functionality is documented
[x] All code style checks pass
[x] New code contribution is covered by automated tests
[x] All new and existing tests pass

wayfair / vsm-ios