feat: add support for custom CacheTagInvalidator

[euaaaio]

This change allows users to provide a custom cache tag invalidator implementation, enabling them to:

• Implement custom invalidation logic (debounce, queue, immediate, etc.)
• Store tags buffer in custom storage (MongoDB, Redis, etc.)
• Add logging, metrics, or monitoring
• Integrate with external invalidation systems

Changes:

• Created CacheTagInvalidator interface with single add(tags) method
• Renamed CacheTagInvalidator class to InMemoryCacheTagInvalidator
• Added cacheTagInvalidator option to MultiCacheServerOptions
• Updated multiCache plugin to use factory pattern for invalidator initialization
• Added comprehensive documentation with MongoDB example
• Updated all tests to use InMemoryCacheTagInvalidator
• Exported CacheTagInvalidator type from module

The API follows factory pattern (similar to cacheTagRegistry):

export default defineMultiCacheOptions(() => {
  return {
    cacheTagInvalidator: (cache, cacheTagRegistry) => {
      return new MyCustomInvalidator(cache, cacheTagRegistry, mongoConnection)
    }
  }
})

[euaaaio]

Oh, sorry. That was my own experiment.