Add the ability to perform signatures in a provider

Add the ability to perform signatures in a provider

This makes EVP_PKEY_sign and EVP_PKEY_sign_init provider aware. It
also introduces the new type EVP_SIGNATURE to represent signature
algorithms. This also automatically makes the EVP_Sign* APIs provider
aware because they use EVP_Digest* (which is already provider aware)
and EVP_PKEY_sign(_init) under the covers.

At this stage there are no signature algorithms in any providers. That
will come in the following commits.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9753)