Add asynchronous metric, FilesystemCacheCapacity - total capacity

Why it matters

This feature provides a global metric for infrastructure monitoring by exposing the total capacity of the cache virtual filesystem, allowing better insights into filesystem cache usage and resource management.

How to use it

Users can access the new asynchronous metric FilesystemCacheCapacity via the existing monitoring systems that support ClickHouse asynchronous metrics. No additional configuration is required to enable this metric; it will appear automatically for global infrastructure monitoring.