Add azblob support to BlobFS

[hairyhenderson]

Fixes #8

Adds support for Azure Blob Storage.

There's a weird quirk in how azblob seems to work... Running fs.ReadDir on it reports two DirEntries - one which has dir mode, and another that doesn't...

This is also obvious with the following command:

$ az storage blob list --delimiter / --account-name azureopendatastorage --container-name citydatacontainer --prefix Crime/Processed/2020/1/20/

One tell-tale sign that a certain item is a directory is that it'll have no ContentType. There is a default otherwise, so for now I'm simply ignoring the file entry with no content type.

As for unit tests, it may not be as simple to test this as it was for s3/gs - I can't find an appropriate fake Azure Blob package. There is "Azurite", but that's not written in Go, and seems designed to run as a separate process. Maybe I can write a decent integration test at some point.

For now I'll test manually with Azure's Open Datasets, and wire it off in CI.

Signed-off-by: Dave Henderson [email protected]

[hairyhenderson]

While I'm not confident that this implementation works flawlessly, I'm going to merge this anyway... In theory it should work pretty well most of the time 😉