[design] Make it possible to use explicitly fetched signature implementation

This design is to allow the use of explicitly fetched EVP_SIGNATURE
implementations.

Ref: openssl/project#171

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22129)