`StorageInstanceManager` leaks internals

[janbuchar]

The StorageInstanceManager class uses a __default__ alias to represent the default storage in its internal cache.

The Apify SDK for Python needs to know this internal alias so that it can open the default storages - see https://github.com/apify/apify-sdk-python/blob/master/src/apify/storage_clients/_apify/_api_client_creation.py#L115-L121. I guess we have a leaky absraction here.

I suggest we add a special method to StorageInstanceManager for opening the default storage.

[Pijukatel]

The same string __default__ is coincidental and comes from the exact same requirement - uncommon storage name that will not be used by the users and that communicates the intent -> special key for default storage. It is not required for both strings to be the same, though.

[janbuchar]

The same string __default__ is coincidental and comes from the exact same requirement - uncommon storage name that will not be used by the users and that communicates the intent -> special key for default storage. It is not required for both strings to be the same, though.

OK then! It would sure be nice not to mention it in docblocks though, shall I replace it?