constants
- Move validation functions from `internal/constants` to `internal/configuration/settings/validation` - Concatenate all OpenVPN constants in `internal/constants/openvpn.go`
- Feat: `MULTIHOP_ONLY` variable - Feat: `COUNTRY` variable - Feat: `CITY` variable - Feat: `REGION` variable, with retro-compatibility - Feat: merge servers from API, zip and hardcoded hostnames - Fix: remove outdated and duplicate servers - Maint: faster update with fully parallel DNS resolutions