Make it possible to easily specify a libctx for EVP_DigestSign*

Make it possible to easily specify a libctx for EVP_DigestSign*

EVP_DigestSignInit_ex and EVP_DigestVerifyInit_ex did not provide the
capability to specify an explicit OPENSSL_CTX parameter. It is still
possible by explicitly setting an EVP_PKEY_CTX - but in most cases it
would be much simpler to just specify it in the Init call. We add the
capability to do that.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11353)