[Storage Cleaner] Add more basic functionality to storage adapters

[2015aroras]

This adds or updates the following functionality for storage adapters:

• is_dir: Create a separate inner method to avoid calls between public-facing methods when the unsharder is implemented.
• download_folder: Download a folder from storage to a local destination. Downloading a file can be done using cached path and so I exclude it.
• upload: Upload a file or folder from local FS to storage.

PR Train:

1. [Storage Cleaner] Add script for removing bad runs from local & cloud storage #364
2. [Storage Cleaner] Migrate to cached_path #378
3. [Storage Cleaner] Add option to get full paths when listing entries #379
4. This PR [Storage Cleaner] Add more basic functionality to storage adapters #380
5. [Storage Cleaner] Add unsharding to storage cleaner #381
6. [Storage Cleaner] Handle some legacy checkpoints in unsharding #382

[epwalsh]

LGTM.

One nit: A lot of the methods take some sort of path argument as a str, and then convert to a Path, while others take PathOrStr. I think it would be cleaner to allow everything to take PathOrStr.

[2015aroras]

LGTM.

One nit: A lot of the methods take some sort of path argument as a str, and then convert to a Path, while others take PathOrStr. I think it would be cleaner to allow everything to take PathOrStr.

The reason I use str for most things is that Path() breaks cloud URLs (it changes // to /). Using PathOrStr for most of the storage adapter methods could encourage the wrong behavior of passing in a Path for cloud paths. Thus I avoid using Path or PathOrStr for paths that could be from the cloud.

Most of the methods that take some a str path argument and convert to Path are in the local FS adapter, where the path is a local path and so doesn't get broken by Path().

[2015aroras]

@epwalsh Sorry, didn't mean to re-request review for this one since it is more or less unchanged (just a different base).