PSA: implement key derivation for finite-field Diffie-Hellman

[gilles-peskine-arm]

Implement psa_key_derivation_output_key to derive finite-field Diffie-Hellman keys.

Prerequisites:

• PSA: implement key derivation for ECC keys #3260 — better resolve the byte/bits redraw issue first.
• FFDH 2a: implement Diffie-Hellman key agreement for RFC 7919 groups #3261 — being able to derive keys is pointless if there's no way to use them.

FFDH is not used much anymore, and this functionality does not exist in the mbedtls_xxx API, so this is low-priority until somebody requests it explicitly.