create correct Capabilities request answer and write maintenance instruction
if the current response works:\
- check if we need a different structure depending on the osclient used, since that fundamentally defines the capabilities
- extend the (initially probably incorrect) ModelData struct in modeldata/modeldata.go (or move it to the client), which should list the modules supported by the gnmitarget. this struct should be used to answer gnmi capabilities requests
- document how to change/expand the struct for future extensions, so the requests can be answered correctly