This PR improves the way the ClickHouse config path,
preprocessed_configs path etc are being joined inside
`ConfigProcessor.cpp`.
Uses `std::filesystem::path` to join the paths instead of
standard string concatenation. This would improve path concatenation.
Current method of concatenation results in:
```bash
➜ clickhouse-server ls -latr
total 68
-rw-r--r-- 1 root root 5587 Aug 31 05:00 users.xml
drwxr-xr-x 143 root root 12288 Sep 7 11:23 ..
lrwxrwxrwx 1 root root 41 Sep 7 11:23 preprocessed -> /var/lib/clickhouse//preprocessed_configs
drwxr-xr-x 2 root root 4096 Sep 7 11:23 users.d
-rw-r--r-- 1 root root 34252 Sep 7 11:28 config.xml
drwxr-xr-x 4 root root 4096 Sep 7 11:28 .
drwxr-xr-x 2 root root 4096 Sep 7 11:28 config.d
```
The symlink `/var/lib/clickhouse//preprocessed_configs` will still work.
amd this PR is an improvement to the current method of path
concatenation.