Support async-only streams

[mycroes]

Async operations are used to avoid blocking threads on IO wait. Unfortunately NPOI currently only supports sync operations when writing documents to streams, which for instance doesn't work with AspNetCore Kestrel defaults that don't permit sync IO. Adding async variants of Write methods that take a Stream would permit consumers to choose between sync or async operation.

I'm willing to look into the changes required for async Writes and submit a PR if you're open to including this.