When there is a single frame with no fields (e.g. splunk datasource) SSE errors when trying to figure out the data type. This frame needs to exist since this is where the executedQueryString metadata exists.
This adds a new return type to SSE to represent no data, so the original frame with its metadata can still be maintained.
* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins
