Add an ECALL for issuing execution tokens

[longtomjr]

• Add ECALL that takes a UUID and access-key along with the token details (use count and binary hash) -> Encrypted payload
• Decrypt the payload
• Lookup UUID (using the state management abstraction) and verify that (depends on: Add an ECALL to save access-key hash and UUID for a data set #37):
  1. The UUID exists
  2. The hash saved alongside the UUID correspond to the hash of the provided access-key
• Create a token and sign it in some way (using an mrenclave+mrsigner unique key)
  • Note: Some design for this is still needed
• Save the token (or some subset) (under the UUID) using the state abstraction
• Encrypt the token for the client and return it

Blocks: #74