Refactor OpenSSL 'DSA' EVP_SIGNATURE to also include DSA+hash composites

(in the code, "sigalg" is used to refer to these composite algorithms,
which is a nod to libcrypto and libssl, where that term is commonly used
for composite algorithms)

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24992)