Blobfile

* Remove `BLOBFILE_BACKENDS` environment variable
* Various internal refactorings
* By default, common errors will not be logged unless a request has been retried enough times

* Attempt to query for subscriptions even less often
* Allow configuring connect and read timeouts through `bf.configure`
* Add configure option `output_az_paths`, set this to `True` to output `az://` paths instead of the `https://` ones
* Add configure option `use_azure_storage_account_key_fallback`, set this to `False` to disable falling back to storage account keys. This is recommended because the storage key fallback confuses users and can result in 429s from the Azure management endpoints.
* Remove generated `.pyi` files. These were for use by `pyright`, but confused PyCharm and are no longer necessary for `pyright`.
* Rename non-public modules to make it clear to `pyright` which symbols are exported.

* Fix for azure credentials when using service principals through azure cli, thanks to hauntsaninja for the PR
* Fix for `bf.listdir` on `az://` paths in the presence of explicit directories, thanks to WuTheFWasThat for the PR

* Add support for `az://` urls, thanks to joschu for the PR. All azure urls output by `blobfile` are still the `https://` format.

* Fix to `bf.isdir()` from hauntsaninja, which didn't work on some unusual azure directories.
* Fewer calls to azure API to reduce chance of hitting rate limits, thanks to hauntsaninja for the PR
* Tokens were being expired at the wrong time, thanks to hauntsaninja for the PR

* Sleep when checking copy status, thanks to hauntsaninja for the PR