Make AzureBlobFileSystem anon behaviour configurable via env var.

[microft]

Making the anon default value configurable from an env var makes integration easier with helm charts that provide features like RBAC in Azure accounts.

In a multi-cloud application I am working on it is impacting that the code needs to change between AWS and Azure.
The default in s3fs is anon=False, in adlfs anon=True.

This PR would allow for configuration only to change without any code change.

It would facilitate integration with https://azure.github.io/azure-workload-identity/docs/quick-start.html without any code change apart from environment variables.

[TomAugspurger]

Thanks for this PR. A few more requests:

1. Add docs for this to the class docstring
2. Add docs in the README
3. Add a test

[TomAugspurger]

Are you able to add a test and docs?

I think pytest has a fixture to monkeypatch environment variables for a test. Just need to confirm that it gets set in __init__ properly.

[TomAugspurger]

Checking in here, will you have a chance to update these last things @microft?

[microft]

Will try to update in the next few days.
(Festivities plus child birth have kept me mostly offline)

[microft]

@TomAugspurger I got a test going, but the cachable property (inherited from AbstractFileSystem) seems to complicate testing multiple instances with the same account_name and connection_string. Hence the monkey patch on cachable for that test.
Hope it makes sense.

[r-priyam]

@TomAugspurger: Any idea when this can be approved and merged, please? It's affecting us badly, too.

[TomAugspurger]

Thanks!