Now that ACVP test vectors exist, support has been added for this mode.
See https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-108r1.pdf
Note that the test vectors used fairly large values for the input key
and the context, so the contraints for these has been increased from
256 to 512 bytes.
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19916)
