Implement auto-purging of storages

[B4nan]

We need the same bahavior as with the JS version:

• crawlee implements the base storage classes
• every async operation checks if it was the first call, and purges automatically unless opted-out via CRAWLEE_PURGE_ON_START env var (with a falsy value like 0 or false)
• we have this method that it called on many places in the storage methods like open or getInput https://crawlee.dev/api/core/function/purgeDefaultStorages
• since the SDK uses those storage classes, it has the same behavior out of box
• internally this works by calling purge method on the storage client, so this also means both memory storage and apify client need to implement this purge method

Related: https://github.com/apify/apify-cli/issues/545

[vdusek]

Closing as it was resolved in https://github.com/apify/crawlee-py/pull/150