Added executable_pool dictionary source and closed issue #14528

Why it matters

This feature introduces a new dictionary source type, executable_pool, which allows users to define dictionaries that fetch data through executables managed in a pool. It enhances flexibility and efficiency when integrating external data sources or custom data fetching logic into ClickHouse dictionaries, solving problems related to dynamic dictionary data retrieval and resource management.

How to use it

To use the executable_pool dictionary source, define a dictionary with source set to executable_pool in the dictionary XML configuration. Specify the executable pool parameters including the path to the executable and pool size. This enables fetching dictionary data by running these executables efficiently in a managed pool.