identity: Add support for ECDSA auth and don't require session encryption.

This adds a new method which allows applications to use mdoc ECDSA
authentication instead of mdoc MAC authentication. Additionally, also
relax requirements on SessionTranscript so the APIs can be used even
when mdoc session encryption isn't being used.

Bug: 241912421
Test: atest VtsHalIdentityTargetTest
Test: atest android.security.identity.cts

Change-Id: I25336f1352102208887531d066ec432a9ae3cd36