Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
new RoleDescriptor.ApplicationResourcePrivileges[] {
                        RoleDescriptor.ApplicationResourcePrivileges.builder()
                            .application("kibana-*")
                            .resources("*")
                            .privileges("reserved_ml_apm_user")
                            .build() },
                    null,
                    null,
                    MetadataUtils.getDeprecatedReservedMetadata("This role will be removed in 8.0"),
                    null
                )
            )
            .put(
                "machine_learning_user",
                new RoleDescriptor(
                    "machine_learning_user",
                    new String[] { "monitor_ml" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".ml-anomalies*", ".ml-notifications*")
                            .privileges("view_index_metadata", "read")
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".ml-annotations*")
                            .privileges("view_index_metadata", "read", "write")
                            .build() },