Plugins: Refactor creation of plugin context to dedicated service (#66451)

* first pass

* fix tests

* return errs

* change signature

* tidy

* delete unnecessary fields from test

* tidy

* fix tests

* simplify

* separate error check in API

* apply nits

pkg/expr/transform.go

@@ -9,6 +9,7 @@ import (
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
 
 	"github.com/grafana/grafana/pkg/services/datasources"
+	"github.com/grafana/grafana/pkg/services/user"
 )
 
 // Request is similar to plugins.DataQuery but with the Time Ranges is per Query.
@@ -17,7 +18,7 @@ type Request struct {
 	Debug   bool
 	OrgId   int64
 	Queries []Query
-	User    *backend.User
+	User    *user.SignedInUser
 }
 
 // Query is like plugins.DataSubQuery, but with a a time range, and only the UID
@@ -136,9 +137,3 @@ func hiddenRefIDs(queries []Query) (map[string]struct{}, error) {
 	}
 	return hidden, nil
 }
-
-func (s *Service) decryptSecureJsonDataFn(ctx context.Context) func(ds *datasources.DataSource) (map[string]string, error) {
-	return func(ds *datasources.DataSource) (map[string]string, error) {
-		return s.dataSourceService.DecryptedValues(ctx, ds)
-	}
-}