DataFrame: split DataFrameHelper into MutableDataFrame and FieldCache (#18795)

* add appending utility

* add appending utility

* update comment

* rename to mutable

* move mutable functions out of DataFrameHelper

* move mutable functions out of DataFrameHelper

* move mutable functions out of DataFrameHelper

* turn DataFrameHelper into FieldCache

* guess time from name

* graph the numbers

* return the timeField, not just the index

* just warn on duplicate field names

* only use a parser if the input is a string

* append init all fields to the same length

* typo

* only parse string if value is a string

* DataFrame: test fixes

* Switch to null for missing values

* Fixed tests

public/app/plugins/datasource/testdata/StreamHandler.ts

@@ -7,7 +7,7 @@ import {
   LoadingState,
   LogLevel,
   CSVReader,
-  DataFrameHelper,
+  MutableDataFrame,
   CircularVector,
   DataFrame,
 } from '@grafana/data';
@@ -182,7 +182,7 @@ export class SignalWorker extends StreamWorker {
     const vals = new CircularVector({ capacity: maxRows });
     this.values = [times, vals];
 
-    const data = new DataFrameHelper({
+    const data = new MutableDataFrame({
       fields: [
         { name: 'Time', type: FieldType.time, values: times }, // The time field
         { name: 'Value', type: FieldType.number, values: vals },
@@ -351,7 +351,7 @@ export class LogsWorker extends StreamWorker {
     const lines = new CircularVector({ capacity: maxRows });
 
     this.values = [times, lines];
-    this.data = new DataFrameHelper({
+    this.data = new MutableDataFrame({
       fields: [
         { name: 'Time', type: FieldType.time, values: times },
         { name: 'Line', type: FieldType.string, values: lines },