add authorized-by item

[nedmsmith]

This captures the discussion thread related to 'authorized-by' element to be added to measurement-map. This will allow measurement values contained in measurement-map to identify the authorized source of valid values. It names one or more (public) keys. The Verifier is expected to locate a credential that authorizes the key(s) (aka the entity(ies) entitled to assert measurement values) for the triples that are included in the tag.

Note: The signer of the RIM is asserting that the authorzied-by entity(ies) are the ones that provided the known good values to the RIM signer. If the RIM signer certificate or other credential type doesn't include a value that acknowledges it as a legitimate RIM signer (for the specific measurements) then the Verifier should not accept the values as valid.

The suggested CDDL is as follows:

measurement-map = {
  ? comid.mkey => $measured-element-type-choice
  comid.mval => measurement-values-map
  ? comid.authorized-by => $crypto-key-type-choice
}

comid.authorized-by = 2

[nedmsmith]

Added issue for authorized-by added to measurement-map