Add an ECALL to save access-key hash and UUID for a data set

[longtomjr]

• Receive and decrypt the payload with the dataset UUID, size, and access-key (payload will come from the data_enclave)
• Hash the access-key https://docs.rs/sodalite/0.4.0/sodalite/fn.hash.html (use sodalite hash SHA-512)
• Hash and save the access-key along with the UUID, and metadata (depends on: Add a module for that abstracts state using SgxFile (in tstd) and serde-json #36)

PR:

• feat: save data access keys #87

Blocks:

• Add an ECALL for issuing execution tokens #38
• Update the data-enclave's upload functionality to persist the upload access key and the using the authorization enclave #39

[PiDelport]

Updated description for the newer hashing approach, and including the dataset size.