rxrpc: rxgk: Provide infrastructure and key derivation

Provide some infrastructure for implementing the RxGK transport security
class:

 (1) A definition of an encoding type, including:

	- Relevant crypto-layer names
	- Lengths of the crypto keys and checksums involved
	- Crypto functions specific to the encoding type
	- Crypto scheme used for that type

 (2) A definition of a crypto scheme, including:

	- Underlying crypto handlers
	- The pseudo-random function, PRF, used in base key derivation
	- Functions for deriving usage keys Kc, Ke and Ki
	- Functions for en/decrypting parts of an sk_buff

 (3) A key context, with the usage keys required for a derivative of a
     transport key for a specific key number.  This includes keys for
     securing packets for transmission, extracting received packets and
     dealing with response packets.

 (3) A function to look up an encoding type by number.

 (4) A function to set up a key context and derive the keys.

 (5) A function to set up the keys required to extract the ticket obtained
     from the GSS negotiation in the server.

 (6) Miscellaneous functions for context handling.

The keys and key derivation functions are described in:

	tools.ietf.org/html/draft-wilkinson-afs3-rxgk-11

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-8-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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