Generalize RKP CDDL schema beyond CSRs

The certificate signing request (CSR) CDDL schema comprises and
authentication wrapper and an inner payload containing details of the
request. Seperate these two parts more clearly in the schema with a view
to reusing the authentication wrapper for other messages.

The change of Csr to be defined in terms of the AuthenticatedMessage
generic type has no effective change on the schema.

A version field is added to CsrPayload, formerly SignedDataPayload, so
that the AuthenticatedMessage and CsrPayload schemas can evolve
independently.

The cert_type field of DeviceInfo is moved up a level into CsrPayload.
This means DeviceInfo only contains device information and not other
fields related to the CSR.

The payload of AuthenticatedMessage is not self-describing. The expected
schema of the payload will be inferred from context, for example the
server endpoint the message is sent to.

Bug: 250910137
Test: n/a - comments only
Change-Id: I2c981ec8fe63995779ce119168ad3d9b40d5b8c5