Working Group Proposal for Reference Types

This is a culmination of many people's work over the last few months to introduce reference types to OCI as part of the [OCI Reference Types WG](oci-ref-type-wg).
This PR contains the changes from our chosen proposal, [Proposal E][proposal-e].

[oci-ref-type-wg]: https://github.com/opencontainers/tob/blob/main/proposals/wg-reference-types.md
[proposal-e]: https://github.com/opencontainers/wg-reference-types/blob/main/docs/proposals/PROPOSAL_E.md

Signed-off-by: Josh Dolitsky <josh@dolit.ski>
Signed-off-by: Brandon Mitchell <git@bmitch.net>
Signed-off-by: Josh Dolitsky <393494+jdolitsky@users.noreply.github.com>
Signed-off-by: Michael Brown <brownxmi@amazon.com>