InitialPayload of ExplicitKeyDiceCertChain

Change the spec for DiceCertChainInitialPayload, removing the map &
directly equating it to bstr .cbor PubKey. Also mandate it to stick to
Core Deterministic Encoding Requirements. The deterministic encoding is
essential to ensure DicePolicies can be applied on the root key.

Test: Builds
Bug: 319613231
Change-Id: I5e12ecbcbae84ae608d784a12f8ae4afc49b5a9d