Add Query Cache support for PostgreSQL

[rahim-kanji]

Issue Description:

Currently, ProxySQL lacks support for query caching for PostgreSQL, a feature that could greatly enhance performance for frequently executed read queries. MySQL has query cache support in ProxySQL, but this functionality is not yet extended to PostgreSQL connections.

Expected Feature:

• Introduce query cache support for PostgreSQL within ProxySQL, configurable via query rules, similar to the existing support for MySQL.
• Cache SELECT queries according to user-defined query rules, allowing selective caching of frequently executed queries based on specific patterns or conditions.
• Provide configurable options for cache size, TTL (Time-To-Live), and invalidation policies.

Benefits:

• Enhanced Performance: Caching the results of frequent SELECT queries can significantly reduce the load on the PostgreSQL backend, leading to faster response times.
• Flexible Control: Using query rules allows database administrators to control which queries are cached, optimizing the cache for the most impactful use cases.
• Resource Efficiency: Reduces CPU and disk I/O overhead by serving cached results for repeated queries, optimizing resource usage on the PostgreSQL server.

Configuration Options:

• Query Rules for Caching: Query rules can define which queries are cached based on patterns, query types, or other conditions.
• Cache Size: Configure memory allocation for query caching.
• TTL (Time-To-Live): Define how long cached query results should persist before expiration.