Fix use of SHAKE as a digest in CMS

Fix use of SHAKE as a digest in CMS

draft-ietf-lamps-cms-sphincs-plus-19 specifies SHAKE as
the message digest algorithm for SLH-DSA-SHAKE-* in CMS.
SHAKE doesn't have a default digest length, so this adds
a SHAKE-specific kludge in CMS.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27087)