Add multiquery_table table transform
Add multiquery_table table transform

The current table transform renders only the first query.
This PR adds a new transform to render all query results in a JOIN-ish
semantic.

* new table transform: Multi-Query table
* columns is the union of all non-value fields
* one value column per query is added
* rows that share all the same label values are merged into one

grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana