Add BigQuery engine specifications (#3193)
Add BigQuery engine specifications (#3193)

As contributed by @mxmzdlv on issue #945
airbnb/superset,airbnb/caravel,timifasubaa/incubator-superset,nekia/incubator-superset-dev,dmigo/incubator-superset,alanmcruickshank/superset-dev,airbnb/superset,mistercrunch/panoramix,nekia/incubator-superset-dev,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,massgov/incubator-superset,mistercrunch/panoramix,dmigo/incubator-superset,massgov/incubator-superset,zhouyao1994/incubator-superset,massgov/incubator-superset,airbnb/superset,FrederichCheng/incubator-superset,airbnb/caravel,FrederichCheng/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,FrederichCheng/incubator-superset,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,timifasubaa/incubator-superset,SingTel-DataCo/incubator-superset,apache/incubator-superset,airbnb/caravel,alanmcruickshank/superset-dev,dmigo/incubator-superset,SingTel-DataCo/incubator-superset,mistercrunch/panoramix,nekia/incubator-superset-dev,SingTel-DataCo/incubator-superset,SingTel-DataCo/incubator-superset,apache/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,massgov/incubator-superset,airbnb/superset,apache/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,mistercrunch/panoramix,dmigo/incubator-superset,FrederichCheng/incubator-superset,apache/incubator-superset,nekia/incubator-superset-dev