Added Functions Filesystemavailable, Filesystemfree, Filesystemcapacity

Why it matters

These functions provide users with the ability to query available, free, and total capacity of the filesystem directly from ClickHouse, enabling better monitoring and management of disk space usage within queries.

How to use it

Use the functions filesystemAvailable(path), filesystemFree(path), and filesystemCapacity(path) by passing the desired filesystem path as an argument to get the respective space metrics.