SQL: Fix SQL dataframe resampling (fill mode + time intervals) (#36937)

* Refactor resample logic

* Adjust test to have one more timestamp out of range

* adjust test + ensure filling

* revert flag flip

* Undo logic - should be timeseries only

* change data calculation based on previous interval

* fix the logics

* fix typo

* fix resample start time, to reuse what sql api returned

* calculate the start point with from truncate by interval

Co-authored-by: Will Browne <will.browne@grafana.com>
Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>
Co-authored-by: Ying WANG <ying.wang@grafana.com>

pkg/tsdb/sqleng/resample_test.go

@@ -230,16 +230,16 @@ func TestResampleWide(t *testing.T) {
 					time.Date(2020, 1, 2, 3, 4, 26, 0, time.UTC),
 				}),
 				data.NewField("Values Ints", nil, []*int64{
+					pointer.Int64(10),
 					pointer.Int64(12),
 					nil,
-					nil,
 					pointer.Int64(15),
 					nil,
 				}),
 				data.NewField(`Values Floats`, data.Labels{"Animal Factor": "sloth"}, []*float64{
+					pointer.Float64(10.5),
 					pointer.Float64(12.5),
 					nil,
-					nil,
 					pointer.Float64(15.0),
 					nil,
 				})),
@@ -257,16 +257,19 @@ func TestResampleWide(t *testing.T) {
 					time.Date(2020, 1, 2, 3, 4, 19, 0, time.UTC),
 					time.Date(2020, 1, 2, 3, 4, 20, 0, time.UTC),
 					time.Date(2020, 1, 2, 3, 4, 24, 0, time.UTC),
+					time.Date(2020, 1, 2, 3, 4, 27, 0, time.UTC),
 				}),
 				data.NewField("Values Ints", nil, []*int64{
 					pointer.Int64(10),
 					pointer.Int64(12),
 					pointer.Int64(15),
+					pointer.Int64(18),
 				}),
 				data.NewField(`Values Floats`, data.Labels{"Animal Factor": "sloth"}, []*float64{
 					pointer.Float64(10.5),
 					pointer.Float64(12.5),
 					pointer.Float64(15.0),
+					pointer.Float64(17.5),
 				})),
 			output: data.NewFrame("wide_test",
 				data.NewField("Time", nil, []time.Time{