[Security Assistant] Add support for manually entering eval datasets (#194072)

## Summary

On cloud environments we don't send the LangSmith credentials through
the `GET /evaluate` route which returns available datasets for
selection, so the datasets are never populated. Since the Dataset field
didn't allow custom options, this means you couldn't perform evals in
cloud environments.

This PR updates the Dataset field to take custom options so that you can
manually enter the dataset name in cloud environments:

<p align="center">
<img width="500"
src="https://github.com/user-attachments/assets/4828e085-180c-42bb-9656-34bda57b74b5"
/>
</p> 


To test, enable the below feature flag to show the evaluation tab under
settings:

```
xpack.securitySolution.enableExperimental:
  - "assistantModelEvaluation"
```

x-pack/packages/kbn-elastic-assistant/impl/assistant/settings/evaluation_settings/evaluation_settings.tsx

@@ -95,6 +95,20 @@ export const EvaluationSettings: React.FC = React.memo(() => {
     },
     [setSelectedDatasetOptions]
   );
+  const onDatasetCreateOption = useCallback(
+    (searchValue: string) => {
+      const normalizedSearchValue = searchValue.trim().toLowerCase();
+      if (!normalizedSearchValue) {
+        return;
+      }
+      const newOption = {
+        label: searchValue,
+      };
+
+      setSelectedDatasetOptions([newOption]);
+    },
+    [setSelectedDatasetOptions]
+  );
 
   // Predictions
   // Connectors / Models
@@ -244,6 +258,7 @@ export const EvaluationSettings: React.FC = React.memo(() => {
             options={datasetOptions}
             selectedOptions={selectedDatasetOptions}
             onChange={onDatasetOptionsChange}
+            onCreateOption={onDatasetCreateOption}
             compressed={true}
           />
         </EuiFormRow>

x-pack/packages/kbn-elastic-assistant/impl/assistant/settings/evaluation_settings/translations.ts

@@ -163,7 +163,8 @@ export const EVALUATOR_DATASET_LABEL = i18n.translate(
 export const LANGSMITH_DATASET_DESCRIPTION = i18n.translate(
   'xpack.elasticAssistant.assistant.settings.evaluationSettings.langsmithDatasetDescription',
   {
-    defaultMessage: 'Name of dataset hosted on LangSmith to evaluate.',
+    defaultMessage:
+      'Name of dataset hosted on LangSmith to evaluate. Must manually enter on cloud environments.',
   }
 );