Add convenience functions EVP_str2ctrl() and EVP_hex2ctrl()

These functions are generalizations of EVP_PKEY_CTX_str2ctrl() and
EVP_PKEY_CTX_hex2ctrl().  They will parse the value, and then pass the
parsed result and length to a callback that knows exactly how to pass
them on to a main _ctrl function, along with a context structure
pointer.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7393)