[Cases] Introduces the incremental_id field and the id incrementer SO (#219757)

## Summary

This is the first PR related to
https://github.com/elastic/kibana/issues/212570. (more up-to-date
information in the [design
doc](https://docs.google.com/document/d/1DZKTPl7UryYjpjVMNhIYbE82OADVOg93-d02f0ZQtUI/edit?tab=t.0))

It only introduces the `incremental_id` field to the cases object and
introduces the new "id incrementer" saved object. Tests and migrations
have been changed accordingly and the `incremental_id` field is removed
from cases import/export.

The motivation behind releasing these changes first is so that
serverless deployments will know about the new field and SO once the
other changes are coming.


### Checklist

- [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

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-check-mappings-update-cli/current_fields.json

@@ -174,6 +174,7 @@
     "external_service.pushed_by.full_name",
     "external_service.pushed_by.profile_uid",
     "external_service.pushed_by.username",
+    "incremental_id",
     "observables",
     "observables.typeKey",
     "observables.value",
@@ -216,6 +217,11 @@
   "cases-connector-mappings": [
     "owner"
   ],
+  "cases-incrementing-id": [
+    "@timestamp",
+    "last_id",
+    "updated_at"
+  ],
   "cases-rules": [
     "counter",
     "createdAt",

packages/kbn-check-mappings-update-cli/current_mappings.json

@@ -600,6 +600,9 @@
           }
         }
       },
+      "incremental_id": {
+        "type": "unsigned_long"
+      },
       "observables": {
         "properties": {
           "typeKey": {
@@ -733,6 +736,20 @@
       }
     }
   },
+  "cases-incrementing-id": {
+    "dynamic": false,
+    "properties": {
+      "@timestamp": {
+        "type": "date"
+      },
+      "last_id": {
+        "type": "keyword"
+      },
+      "updated_at": {
+        "type": "date"
+      }
+    }
+  },
   "cases-rules": {
     "dynamic": false,
     "properties": {