Expose more functionality from hwtrust

Instead of having parsing code in remote_prov_utils and hwtrust, we
should expose more of the values that are parsed in hwtrust and use
that interface to implement the verification of a CSR.

The aim of this CL is to preserve the functionality that previously
existed in remote_prov_utils and, also, add more tests.

Test: atest libkeymint_remote_prov_support_test
Change-Id: Id5408a425f28ea99052ba954c34441ed9307a5d2