[Response Ops][Rules] Rule Model Version Forward Compat (#193233)

## Summary

This PR implements the `forwardCompatibility` field for the rule model
version in preparation for the flapping PR
(#190019). We're doing this so we
have the point to revert back to if we need to where the
`forwardCompatibility` is defined.

---------

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

src/core/server/integration_tests/ci_checks/saved_objects/check_registered_types.test.ts

@@ -59,7 +59,7 @@ describe('checking migration metadata changes on all registered SO types', () =>
         "action": "0e6fc0b74c7312a8c11ff6b14437b93a997358b8",
         "action_task_params": "b50cb5c8a493881474918e8d4985e61374ca4c30",
         "ad_hoc_run_params": "d4e3c5c794151d0a4f5c71e886b2aa638da73ad2",
-        "alert": "3a67d3f1db80af36bd57aaea47ecfef87e43c58f",
+        "alert": "e920b0583b1a32338d5dbbbfabb6ff2a511f5f6c",
         "api_key_pending_invalidation": "1399e87ca37b3d3a65d269c924eda70726cfe886",
         "apm-custom-dashboards": "b67128f78160c288bd7efe25b2da6e2afd5e82fc",
         "apm-indices": "8a2d68d415a4b542b26b0d292034a28ffac6fed4",

x-pack/plugins/alerting/server/saved_objects/model_versions/rule_model_versions.ts

@@ -12,6 +12,7 @@ export const ruleModelVersions: SavedObjectsModelVersionMap = {
   '1': {
     changes: [],
     schemas: {
+      forwardCompatibility: rawRuleSchemaV1.extends({}, { unknowns: 'ignore' }),
       create: rawRuleSchemaV1,
     },
   },