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

Co-authored-by: Josh Dolitsky <[email protected]>
Co-authored-by: Josh Dolitsky <[email protected]>
Co-authored-by: Michael Brown <[email protected]>
Signed-off-by: Brandon Mitchell <[email protected]>