Add incremental_sync feature flag to Connector index (#158641)

# Part of https://github.com/elastic/enterprise-search-team/issues/4629

## Summary

This PR adds the feature flag `incremental_sync` to index
`.elastic-connectors`.

This PR also adds `sync_rules` feature flag, which I believed was missed
out previously.


### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/plugins/enterprise_search/server/index_management/setup_indices.test.ts

@@ -48,6 +48,25 @@ describe('Setup Indices', () => {
         properties: {
           filtering_advanced_config: { type: 'boolean' },
           filtering_rules: { type: 'boolean' },
+          incremental_sync: {
+            properties: {
+              enabled: { type: 'boolean' },
+            },
+          },
+          sync_rules: {
+            properties: {
+              basic: {
+                properties: {
+                  enabled: { type: 'boolean' },
+                },
+              },
+              advanced: {
+                properties: {
+                  enabled: { type: 'boolean' },
+                },
+              },
+            },
+          },
         },
       },
       filtering: {

x-pack/plugins/enterprise_search/server/index_management/setup_indices.ts

@@ -37,6 +37,25 @@ const connectorMappingsProperties: Record<string, MappingProperty> = {
     properties: {
       filtering_advanced_config: { type: 'boolean' },
       filtering_rules: { type: 'boolean' },
+      incremental_sync: {
+        properties: {
+          enabled: { type: 'boolean' },
+        },
+      },
+      sync_rules: {
+        properties: {
+          basic: {
+            properties: {
+              enabled: { type: 'boolean' },
+            },
+          },
+          advanced: {
+            properties: {
+              enabled: { type: 'boolean' },
+            },
+          },
+        },
+      },
     },
   },
   filtering: {