Unified Storage: in SQL template, also handle output data, improve API, examples and docs (#87560)

* preview of work so far

* stylistic improvements

* fix linters

* remove golden tests, they may cause the system to be too rigid to changes

* remove unnecessary code for golden tests

* remove white space mangling in Execute

* also handle output data, improve API, examples and docs

* add helper methods

* fix interface

pkg/services/store/entity/sqlstash/sqltemplate/into_test.go

@@ -0,0 +1,36 @@
+package sqltemplate
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestScanDest_Into(t *testing.T) {
+	t.Parallel()
+
+	var d ScanDest
+
+	colName, err := d.Into(reflect.Value{}, "some field")
+	if colName != "" || err == nil || len(d.GetScanDest()) != 0 {
+		t.Fatalf("unexpected outcome, got colname %q, err: %v, scan dest: %#v",
+			colName, err, d)
+	}
+
+	data := struct {
+		X int
+		Y byte
+	}{}
+	dataVal := reflect.ValueOf(&data).Elem()
+
+	colName, err = d.Into(dataVal.FieldByName("X"), "some int")
+	if err != nil || colName != "some int" || len(d) != 1 || d[0] != &data.X {
+		t.Fatalf("unexpected outcome, got colname %q, err: %v, scan dest: %#v",
+			colName, err, d)
+	}
+
+	colName, err = d.Into(dataVal.FieldByName("Y"), "some byte")
+	if err != nil || colName != "some byte" || len(d) != 2 || d[1] != &data.Y {
+		t.Fatalf("unexpected outcome, got colname %q, err: %v, scan dest: %#v",
+			colName, err, d)
+	}
+}