filesystem_caches_path in the configuration should be defined by default

[alexey-milovidov]

1. Allow relative paths for the value of filesystem_caches_path. It should be relative to path.
2. Set it to cache by default in clickhouse-server's configuration to avoid a minor vulnerability with creating tables with dynamic disks.
3. Make sure it applies to dynamic disks, but statically configured paths are still allowed to point anywhere.
4. Make sure clickhouse-local still allows dynamic disks to have paths point anywhere.
5. Update the code of ClickBench, related to clickhouse-web.

[kssenii]

As filesystem_caches_path affects not only dynamic disks, but all disks, I'd rather not set it in default config, but do this #58869 (See PR description).