phoebus-scan-server: nixos module

[Synthetica9]

Closes #74

[minijackson]

Seems to work great, thanks a lot! Could you add some documentation about it? Something that covers at least the "default" usecase, for example like this: https://github.com/epics-extensions/EPNix/blob/master/doc/nixos/guides/phoebus-save-and-restore.md

If you have some, some example config / link to upstream documentation would be helpful, but I couldn't find something other than these two files:

• https://github.com/ControlSystemStudio/phoebus/blob/v4.7.3/services/scan-server/src/main/resources/examples/scan_config.xml
• https://github.com/ControlSystemStudio/phoebus/blob/v4.7.3/services/scan-server/src/main/resources/config/scan_config.xsd