Improve Use of MRSIGNER Value to Authenticate for Attestation

[bit-bender]

Revise the check of the peer enclave's MRSIGNER value for authentication on the attestation interface.

Currently, the implementation checks that both enclaves were signed with the same key.

A more flexible approach would be to instead compare the peer's MRSIGNER value to a known trusted cert file.

In order to do this, we would need to update the CLI to specify the cert file, figure out how to convert it to an MRSIGNER value, and then get the computed MRSIGNER value into the enclave to do the comparison.