Implement write and delete for S3FileSystem

[wjones127]

As part of prototyping apache/arrow-datafusion#2246 I am implementing the API for S3 to see what it's like to implement these operations for object stores. (So far, I've learned that AsyncWrite is quite difficult to implement 😢.)

[matthewmturner]

Very happy to see this - i had it on my to-do list as well. What issues have you run into with AsyncWrite?

[wjones127]

Basically took me a bit to figure out how to create this state enum:

enum S3MultipartUploadState {
    Ready,
    UploadInProgress(Pin<Box<dyn Future<Output = Result<()>>>>),
    CompleteInProgress(Pin<Box<dyn Future<Output = Result<()>>>>),
    Completed
}

Need to be able to hold onto the futures between the different poll calls. Hopefully, with an example for S3, the implementations for other object stores can just follow the same general pattern.