XD-1811 Custom module registry location

- In addition to `xd.module.home` and `classpath:/modules` as the default
module registry locations, an additional `xd.customModule.home` is provided
to let the user define any specific supported URL path for the module registry.
- Add an entry for `xd.customModule.home` in servers.yml so that user can
specify it.
Technically, we need to be able to scan the location, so http:// will not work, so better not mention it