Keeper Improvement: Add createIfNotExists Command

Why it matters

This feature introduces a new createIfNotExists command to Keeper, allowing users to create nodes only if they do not already exist. It helps prevent errors or conflicts during node creation in distributed coordination, improving robustness and ease of use.

How to use it

Users can utilize the new createIfNotExists command via Keeper's API or client when creating nodes. This command checks for the existence of a node and creates it only if it is absent, avoiding exceptions related to duplicate nodes.