We plan to identify AIKs based on their public key. The raw blob should
be available via the AttestationParameters, but we hope that users will
only use that struct for generating challenges.
Because this parses the public key on AIK creation and loading, this PR
should have existing coverage.
* Refactor serialized keys into own structure, in preparation for making Key an interface.
* Refactor key logic into separate structures for each platform/TPMversion invariant.
* Implement review feedback
