[WIP] Add an implementation of ReadAsync to ZipAESStream

[Numpsy]

refs #572

Add an implementation of ReadAsync to ZipAESStream, with associated StreamUtils change and a copy/pasted unit test (that currently fails on .NETCore 3.1 due to #572).

WIP because:

1. There is still a bunch of duplication between ReadAndTransform/ReadAndTransformAsync, and the copy/pasted signature of TransformFromSlideBuffer doesn't feel ideal.

2. Looking at ZipFile.GetInputStream a bit more, I don't think it will actually do async reads from the base stream unless PartialInputStream is given some async support as well, so the current code might not actually be any 'more' async that just making ReadAsync call Read synchrounously and then returning the result.

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[Numpsy]

just making ReadAsync call Read synchrounously and then returning the result

which would be the fastest change if we want a fix ASAP, but not as nice as a real async impl, which fits in more with the other async supporting changes.