enterprise.platformKeys api: Register machine key

As part of a feature to side-load machine keys verified by remote
attestation into the system-wide token using the
enterprise.platformKeys.challengeMachineKey API, the
SignEnterpriseChallenge method needs another argument that specifies the
key name used for SignedPublicKeyAndChallenge.

When challenging machine key with register = true, the EMK cannot be
registered as that would relinquish it and the DMServer relies on it to
remain stable.

This CL generates a new machine key to side-load into the system-wide
token in this case. The key will be used for SignedPublicKeyAndChallenge
but the challenge response will still be singed using the stable EMK.

Depends on CL:1714597.

BUG=b:35580115
TEST=manually tested,
unit_tests --gtest_filter=EPKChallengeMachineKeyTest.KeyRegisteredSuccess

Change-Id: I4c426f06d1ff32333bf1383a54d4840f119aeeeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708948
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686730}