system_api: New fields and messages for challenge-response cryptohome keys

Adds support for challenge-response cryptohome keys which is based on RSA
signing keys:
* Extend KeyData with a new type enum and new fields that describe public
  information of an RSA key;
* Add KeyChallengeRequest that will be a parameter for the new
  KeyChallenge signal and will hold the challenge blob and the reference
  to the challenged RSA key;
* Add KeyChallengeResponse that will be a parameter for the new
  RespondKeyChallenge method and will hold the RSA signature.

BUG=chromium:806788
TEST=none (verify that protos compile successfully)

Change-Id: I0c7168668daf7bb22509962ce569235a2a5ac381
Reviewed-on: https://chromium-review.googlesource.com/911797


Commit-Ready: Maksim Ivanov <emaxx@chromium.org>
Tested-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>