Support relative path when running as Windows Service

OS: Windows

Issue
in `AdGuardHome.yml`, there is an option `upstream_dns_file`, which serves as an extension of the yaml config.
Usually I put the upstream_dns_file in the same folder , in other words, a relative path ref such as "upstream_dns_file: upstream_dns_file.txt" will do the trick when ADGuardHome is running as an exe file

However, when running as a Windows Service, ADGuardHome wont recognize such relative path ref, In other words, when running as a windows service, only absolute path such as `upstream_dns_file: D:\AdGuardHome\ups_domains.conf` will work

Can we make the Service part supports rel paths ? Thanks