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
.resources("*")
                    .privileges("all")
                    .build() },
            null,
            null,
            MetadataUtils.DEFAULT_RESERVED_METADATA,
            null
        );
    }
    private static RoleDescriptor kibanaAdminUser(String name, Map<String, Object> metadata) {
        return new RoleDescriptor(
            name,
            null,
            null,
            new RoleDescriptor.ApplicationResourcePrivileges[] {
                RoleDescriptor.ApplicationResourcePrivileges.builder()
                    .application("kibana-.kibana")
                    .resources("*")
                    .privileges("all")
                    .build() },
            null,
            null,
            metadata,
            null
        );
    }
    public static RoleDescriptor kibanaSystemRoleDescriptor(String name) {
        return new RoleDescriptor(
            name,
            new String[] {