[Feature]: Distributed Cache Support for Query Caching

[eneshoxha]

Title: Add IDistributedCache-based query caching behavior

Labels: enhancement, mediator

Body:

Problem

CachingQueryBehavior<TQuery, TResult> only supports IMemoryCache. In distributed systems (microservices, multi-instance deployments, Cortex.Streams scenarios), query results often need to be cached in Redis, SQL Server, or another distributed cache.

Proposed Solution

Create a Cortex.Mediator.Behaviors.DistributedCaching package (or add to core) with:

• DistributedCachingQueryBehavior<TQuery, TResult> using IDistributedCache
• ICacheSerializer interface for pluggable serialization (default: System.Text.Json)
• Support for the existing ICacheableQuery and [Cacheable] attribute opt-in mechanisms
• DistributedCachingOptions for default TTL, serialization settings, key prefix

services.AddMediatorDistributedCaching(options =>
{
    options.DefaultAbsoluteExpiration = TimeSpan.FromMinutes(10);
    options.Serializer = new JsonCacheSerializer();
});

Relationship to Existing Caching

This complements (not replaces) the existing IMemoryCache-based behavior. Users may use both (L1 in-memory + L2 distributed) via pipeline behavior ordering.