[Proposed] Case Appendix

A Case appendix provides additional data which can help the platform optimize for the structure of how cases are used in the sandbox.

Platforms are welcome to process appendix blocks at any time after receiving them. They do not need to be executed before processing other data in the restore (IE: The restore can be considered successful after appendix block is received and queued, rather than processed).

<appendix xmlns="http://commcarehq.org/appendix/v1" id="" hash="">
    <case case_type="">        <!--- 0 or More: information about a case model
                                     @case_type - Optional -->
        <index property=""/>   <!--- 0 or More: Provides an indication that the id'd property will be frequently used to filter data, and should ideally be optimized for lookups in the platform --->
    </case>
</appendix >