Add serialization for SHA-2 digest contexts

Add serialization for SHA-2 digest contexts

This commit introduces the ability to serialize and deserialize the internal
state of SHA-2 digest contexts (SHA-256 and SHA-512 families).

This functionality is exposed via the new OSSL_DIGEST_SERIALIZATION parameter,
which can be used with EVP_MD_CTX_get_params() to retrieve the state and with
EVP_DigestInit_ex2() to restore it into a new context.

This allows an application to save the state of a hash operation and resume it
later, which is useful for process migration or for saving the state of long-
unning computations. A new test case has been added to verify this.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28837)