When disk name is not specified, it should use the default disk.
They should return max UInt64 for "infinite" virtual filesystems.

**Use case**

ClickHouse cloud.
#include <Functions/IFunction.h>
#include <Functions/FunctionFactory.h>
#include <DataTypes/DataTypesNumber.h>
#include <Interpreters/Context.h>
#include <filesystem>
#include <Poco/Util/AbstractConfiguration.h>
namespace DB
{
namespace
{
struct FilesystemAvailable
{
    static constexpr auto name = "filesystemAvailable";
    static std::uintmax_t get(const std::filesystem::space_info & spaceinfo) { return spaceinfo.available; }
};
struct FilesystemFree
{
    static constexpr auto name = "filesystemFree";
    static std::uintmax_t get(const std::filesystem::space_info & spaceinfo) { return spaceinfo.free; }
};
struct FilesystemCapacity
{
    static constexpr auto name = "filesystemCapacity";
    static std::uintmax_t get(const std::filesystem::space_info & spaceinfo) { return spaceinfo.capacity; }
};
template <typename Impl>
class FilesystemImpl : public IFunction