feat: azure backend

[machow]

Initial implementation of azure backend.

TODO:

• : should I use use_listings_cache=False, rather than listings_expiry_time=0? (yes!)
• clean up, decide how we refer to azure in tests
• fix constructor tests, which try to use the fsspec protocol to figure out the constructor name (e.g. s3 -> board_s3, but azure's protocol is e.g. abfs, while the constructor is board_azure)
• take a quick crack at dircache.__iter__ fails if an item in the cache is expired fsspec/filesystem_spec#987. (We resolved by using use_listings_cache=False). edit: opened a PR (fix dircache iter changing size (#987) fsspec/filesystem_spec#1027), which this PR does not need to wait on.

[machow]

It seems like we could use these rules for naming:

• pytest mark: fs_<protocol>, where <protocol> is an fsspec protocol name. For example s3, gcs, abfs. Note that some have aliases (e.g. az and abfs both produce abfs; azure blob file system). In that case, let's use whichever becomes the value of board.fs.protocol.
• constructor name: board_<r_pins_name>. We can do whatever R pins does for cross compatibility.

For consistency:

• When there are multiple protocol name options, let's always use the one we use for the pytest mark.

[machow]

TODO: look up azure credentials, move into password manager