Structured Measurement Basics

[evgenyp67]

Changing the measurement from being hard-coded to the 3 required files (or previously, and like other Enclaves, memory-layout measurements) to flexible measurement composed of an unlimited number of file hashes.

Files can either be already present in the enclave (resident), or declared to have a certain hash but not yet there (absent).

Files can either be identity -- be a part of the sealing key because they are relevant to what the enclave is, or non-identity. This doesn't affect the attestation measurement, because attestation ensures that the exact desired state is honored by the untrusted host.

TODO:

1. get changes reviewed
2. look into minor todo's
3. improve PR description

Other PR's:

1. Dynamic Measurement: Dynamic Measurement #436

Future PR TODO's:

1. have filesystem calls get files from memory and use the allowlist
2. example with an extra non-identity file (i.e. arg) not affecting the sealing key
3. cmd tool to go from eapp binary to .ke, and a dynamic verifier.
4. dynamic library support