[Enterprise Search] Update ML inference field config UI (#162598)

## Summary

Minor updates to ML inference multi-field configuration UI (no
functional changes):
- Add spacer between field selector and selected field list
- Rename labels
- ~Remove "Actions" label (since there's only a single action)~

![Screenshot 2023-07-26 at 17 08
28](48505ac1-69f3-4c89-960b-68bb510ed830)

x-pack/plugins/enterprise_search/public/applications/enterprise_search_content/components/search_index/pipelines/ml_inference/configure_fields.tsx

@@ -77,7 +77,14 @@ export const ConfigureFields: React.FC = () => {
       <EuiForm component="form">
         {isTextExpansionModelSelected ? (
           <>
-            {areInputsDisabled || <MultiFieldMapping />}
+            {areInputsDisabled ? (
+              <></>
+            ) : (
+              <>
+                <MultiFieldMapping />
+                <EuiSpacer size="l" />
+              </>
+            )}
             <SelectedFieldMappings isReadOnly={areInputsDisabled} />
           </>
         ) : (

x-pack/plugins/enterprise_search/public/applications/enterprise_search_content/components/search_index/pipelines/ml_inference/multi_field_selector.test.tsx

@@ -180,6 +180,10 @@ describe('SelectedFieldMappings', () => {
 
     expect(wrapper.find(EuiBasicTable)).toHaveLength(1);
     const table = wrapper.find(EuiBasicTable);
-    expect(table.prop('columns').map((c) => c.name)).toEqual(['Source field', '', 'Target field']);
+    expect(table.prop('columns').map((c) => c.name)).toEqual([
+      'Source text field',
+      '',
+      'Target field',
+    ]);
   });
 });

x-pack/plugins/enterprise_search/public/applications/enterprise_search_content/components/search_index/pipelines/ml_inference/multi_field_selector.tsx

@@ -143,7 +143,7 @@ export const MultiFieldMapping: React.FC = () => {
             label={i18n.translate(
               'xpack.enterpriseSearch.content.indices.pipelines.addInferencePipelineModal.steps.fields.sourceFieldLabel',
               {
-                defaultMessage: 'Source field',
+                defaultMessage: 'Source text field',
               }
             )}
             helpText={i18n.translate(
@@ -233,7 +233,7 @@ export const SelectedFieldMappings: React.FC<SelectedFieldMappingsProps> = ({ is
       name: i18n.translate(
         'xpack.enterpriseSearch.content.indices.pipelines.addInferencePipelineModal.steps.fields.fieldMappings.sourceFieldHeader',
         {
-          defaultMessage: 'Source field',
+          defaultMessage: 'Source text field',
         }
       ),
     },

x-pack/plugins/enterprise_search/public/applications/enterprise_search_content/components/search_index/pipelines/ml_inference/single_field_selector.tsx

@@ -68,7 +68,7 @@ export const SingleFieldMapping: React.FC = () => {
             label={i18n.translate(
               'xpack.enterpriseSearch.content.indices.pipelines.addInferencePipelineModal.steps.fields.sourceFieldLabel',
               {
-                defaultMessage: 'Source field',
+                defaultMessage: 'Source text field',
               }
             )}
             error={isEmptySourceFields && <NoSourceFieldsError />}