Opserver.Core: add netstandard2.0 build
Opserver.Core: add netstandard2.0 build

manesiotise/Opserver,manesiotise/Opserver,mqbk/Opserver,mqbk/Opserver,opserver/Opserver,opserver/Opserver,manesiotise/Opserver,opserver/Opserver,GABeech/Opserver,GABeech/Opserver