Pass CoRIM-specific context to extractors

[thomas-fossati]

Currently (d14e5af), the reference value and TAs extractors work on a triple level.

type IExtractor interface {
  RefValExtractor(comid.ValueTriples) ([]*handler.Endorsement, error)
  TaExtractor(comid.KeyTriple) (*handler.Endorsement, error)
  SetProfile(string)
}

The profile is the only variable that is made available from the global context.

Other information may become relevant at appraisal time and should be preserved, e.g.:

• CoMID and CoRIM identifiers
• notBefore/notAfter associated with the CoRIM manifest
• CoRIM signer-related information

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Another assumption backed into this interface is that ref-vale and trust anchors are only extracted from certain CoMID triples. This may not hold in two ways: other CoMID triples may be used, or other tags (e.g., CoTS) could be used. While the former is a bit speculative, the latter is probably what we want to have for CCA and PSA going forward.

This likely deserves a different tracking issue.