Attest: Add test that verify and decode individual claims

This consists of three sub-parts: 1) an implementation
of COSE signature verifcation, 2) an implementaiton of
token decoding, 3) test that do comparison to expected
values.

Change-Id: I37dfebf97a4658741ff0721a5b769781b2634ea7
Signed-off-by: Laurence Lundblade <lgl@securitytheory.com>
Co-authored-by: Tamas Ban <tamas.ban@arm.com>