SQL Expressions: Add endpoint to get Schemas (#108864)

Return the SQL schema for all DS queries in request (to provide information to AI / Autocomplete for SQL expressions).

All DS queries are treated as if they were inputs to SQL expressions in terms of conversion, regardless if they are selected in a query or not.

Requires feature toggle queryService = true

Endpoint is apis/query.grafana.app/v0alpha1/namespaces/default/sqlschemas

---------

Co-authored-by: Todd Treece <360020+toddtreece@users.noreply.github.com>

pkg/expr/sql/frame_table.go

@@ -30,7 +30,7 @@ func (ft *FrameTable) String() string {
 	return ft.Name()
 }
 
-func schemaFromFrame(frame *data.Frame) mysql.Schema {
+func SchemaFromFrame(frame *data.Frame) mysql.Schema {
 	schema := make(mysql.Schema, len(frame.Fields))
 
 	for i, field := range frame.Fields {
@@ -48,7 +48,7 @@ func schemaFromFrame(frame *data.Frame) mysql.Schema {
 // Schema implements the mysql.Table interface
 func (ft *FrameTable) Schema() mysql.Schema {
 	if ft.schema == nil {
-		ft.schema = schemaFromFrame(ft.Frame)
+		ft.schema = SchemaFromFrame(ft.Frame)
 	}
 	return ft.schema
 }