[Storage] Change content iterating in _download_async.py, add decompression live tests against compressed dataset

[vincenttran-msft]

• Change from response.load_body() and response.body() in _download_async.py to access content, as this method does not respect the decompress param passed to download_blob API
  • This was resolved by using the iterator directly.
  • An if-else block was added to handle the general hotpath and the validate_content=True path (where the body would have been loaded previously for MD5 check)
• Test cases were added for both sync and async for the compressed data + decompress=T/F scenario.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[vincenttran-msft]

/azp run python - storage - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).