Give all acces to .slo-observability.* indice to kibana user (#97539)

Resolves: https://github.com/elastic/kibana/issues/161585

## Summary

This PR grants `all` privileges on the `.slo-observability.*` indices
used by the observability SLO solution to the `kibana_system` role. 

If there is some convention for naming kibana internal indices, please
let me know.

docs/changelog/97539.yaml

@@ -0,0 +1,5 @@
+pr: 97539
+summary: "Give all acces to .slo-observability.* indice to kibana user"
+area: Authentication
+type: enhancement
+issues: []

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/KibanaOwnedReservedRoleDescriptors.java

@@ -299,7 +299,10 @@ class KibanaOwnedReservedRoleDescriptors {
                 RoleDescriptor.IndicesPrivileges.builder()
                     .indices("logs-cloud_defend.*", "metrics-cloud_defend.*")
                     .privileges("read", "view_index_metadata")
-                    .build() },
+                    .build(),
+                // SLO observability solution internal indices
+                // Kibana system user uses them to read / write slo data.
+                RoleDescriptor.IndicesPrivileges.builder().indices(".slo-observability.*").privileges("all").build() },
             null,
             new ConfigurableClusterPrivilege[] {
                 new ConfigurableClusterPrivileges.ManageApplicationPrivileges(Set.of("kibana-*")),

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStoreTests.java

@@ -595,7 +595,8 @@ public class ReservedRolesStoreTests extends ESTestCase {
             ReservedRolesStore.PREVIEW_ALERTS_INDEX_ALIAS + randomAlphaOfLength(randomIntBetween(0, 13)),
             ReservedRolesStore.PREVIEW_ALERTS_BACKING_INDEX_ALIAS + randomAlphaOfLength(randomIntBetween(0, 13)),
             ReservedRolesStore.LISTS_INDEX + randomAlphaOfLength(randomIntBetween(0, 13)),
-            ReservedRolesStore.LISTS_ITEMS_INDEX + randomAlphaOfLength(randomIntBetween(0, 13))
+            ReservedRolesStore.LISTS_ITEMS_INDEX + randomAlphaOfLength(randomIntBetween(0, 13)),
+            ".slo-observability." + randomAlphaOfLength(randomIntBetween(0, 13))
         ).forEach(index -> assertAllIndicesAccessAllowed(kibanaRole, index));
 
         // read-only index access, including cross cluster