Support an aiobotocore instrumentor

[jamesh1999]

I'm keen to swap botocore/boto3 out for aiobotocore in one of my projects. This is an async alternative but the interface is otherwise almost identical to botocore. One sticking point, however, is a current dependence on the botocore instrumentor.

It's not very difficult to modify the existing instrumentor to work (add a couple of async / await keywords & patch the AioBaseClient class instead of botocore's BaseClient). I wondered if this could be provided alongside the existing botocore instrumentor so that internal changes to the instrumentor don't break things for me downstream.