added etsi rest code, added generation command in Makefile

Makefile

api/go/rest/etsi/.gitignore

api/go/rest/etsi/.openapi-generator-ignore

api/go/rest/etsi/.openapi-generator/FILES

api/go/rest/etsi/.openapi-generator/VERSION

api/go/rest/etsi/.travis.yml

api/go/rest/etsi/README.md

api/go/rest/etsi/api/openapi.yaml

api/go/rest/etsi/api_etsi14_service.go

api/go/rest/etsi/client.go

api/go/rest/etsi/configuration.go

api/go/rest/etsi/docs/DanetetsiStatus.md

api/go/rest/etsi/docs/ETSI14ServiceAPI.md

api/go/rest/etsi/docs/ETSI14ServiceGetKeyRequest.md

api/go/rest/etsi/docs/ETSI14ServiceGetKeyWithIDsRequest.md

api/go/rest/etsi/docs/EtsiError.md

api/go/rest/etsi/docs/EtsiExtensionMandatory.md

api/go/rest/etsi/docs/EtsiExtensionOptional.md

api/go/rest/etsi/docs/EtsiGetKeyResponse.md

api/go/rest/etsi/docs/EtsiGetKeyWithIDsResponse.md