[design] Functions for explicitly fetched signature algorithms

[design] Functions for explicitly fetched signature algorithms

This design goes into more details what was outlined in the design for
[fetching composite (PKEY) algorithms and using them].

It also changes what functionality will be used for this.  The design for
signature was originally to add modified initializers for DigestSign and
DigestVerify, but recent OTC discussions redirected us to have a closer look
at EVP_PKEY_sign() and EVP_PKEY_verify().

[fetching composite (PKEY) algorithms and using them]:
    ./fetching-composite-algorithms.md

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22672)