Select: Allow custom value for selects (#19775)

* WIP: simple poc of allow custom value for selects

* Add support for custom value in segment

* Update snapshots

public/app/plugins/datasource/prometheus/components/__snapshots__/PromQueryEditor.test.tsx.snap

@@ -68,6 +68,7 @@ exports[`Render PromQueryEditor with basic options should render 1`] = `
         Resolution
       </div>
       <Select
+        allowCustomValue={false}
         autoFocus={false}
         backspaceRemovesValue={true}
         className=""
@@ -134,6 +135,7 @@ exports[`Render PromQueryEditor with basic options should render 1`] = `
         Format
       </div>
       <Select
+        allowCustomValue={false}
         autoFocus={false}
         backspaceRemovesValue={true}
         className=""