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
)
            .put(KibanaSystemUser.ROLE_NAME, kibanaSystemRoleDescriptor(KibanaSystemUser.ROLE_NAME))
            .put(
                "logstash_system",
                new RoleDescriptor(
                    "logstash_system",
                    new String[] { "monitor", MonitoringBulkAction.NAME },
                    null,
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(
                "beats_admin",
                new RoleDescriptor(
                    "beats_admin",
                    null,
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder().indices(".management-beats").privileges("all").build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(
                UsernamesField.BEATS_ROLE,
                new RoleDescriptor(
                    UsernamesField.BEATS_ROLE,
                    new String[] { "monitor", MonitoringBulkAction.NAME },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()