KeyValueStore: Method to check key-value store record exists

[drobnikj]

It is impossible to check if a key-value record exists, and users use kvs.getRecord instead, which is inefficient.

The implementation is simple: to check if a record exists, you can use an HTTP request with the method HEAD get Record API URL.

Method: keyValueStoreClient.exists(key: str): bool which returns true or false based on the record.