Adding Interoperable RA-TLS support to RATS-TLS

[imlk0]

This is the task list we need to complete in order to implement the Interoperable RA-TLS proposal on RATS-TLS.

• Initial support for Interoperable RA-TLS X.509 DICE certificate format, with evidence extension. Support new interoperable RA-TLS X.509 DICE cert format #123
• Add support for endorsements extension. Support for DICE endorsements extension #135
• Put <pubkey-hash> in claims-buffer instead of userdata field of evidence. Unify the format of evidence_buffer #144
• Write a document of the steps required by compatibility tests between RATS-TLS and other projects.
• Support endorsements extension for occlum mode. How to generate and verify SGX DCAP collateral in Occlum? occlum/occlum#1102
• Support minimal set of hash functions: sha-256 (ID 1), sha-384 (ID 7) and sha-512 (ID 8) rats-tls/crypto_wrappers: support minimal set of hash functions #147