Add ecall to request execution using an encrypted execution token and some execution parameters

[longtomjr]

• Decrypt the execution token
• Validate that the hash of the token corresponds to a valid function
• Perform local attestation with the data enclave
• Request data from the data enclave with the execution token over the protected channel (established using local attestation) (OCall) [Allocate a buffer with the size specified in the execution token (with some sensible upper max)]
• Decrypt data received from data enclave and pass it to the function that matches the function hash in the execution token
• Return the function result back to the caller
  Note: just ignore the execution parameters for now

Blocked by: #73 and #70