Allow generating presigned URLs for KVS records

Since #453 we can generate presigned URLs for **KVS keys** and **Dataset items**.

JS Client can generate presigned URLs for **separate KVS records** (see [`getRecordPublicUrl`](https://github.com/apify/apify-client-js/blob/ec6e63815299a396a7e8385318a8630635a004ad/src/resource_clients/key_value_store.ts#L94)). This method seems to be missing from the Python client.