Expressions: Move GEL into core as expressions (#29072)

* 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

pkg/tsdb/sqleng/sql_engine.go

@@ -55,8 +55,8 @@ var engineCache = engineCacheType{
 
 var sqlIntervalCalculator = tsdb.NewIntervalCalculator(nil)
 
-//nolint:gocritic
 // NewXormEngine is an xorm.Engine factory, that can be stubbed by tests.
+//nolint:gocritic
 var NewXormEngine = func(driverName string, connectionString string) (*xorm.Engine, error) {
 	return xorm.NewEngine(driverName, connectionString)
 }