crypto: sha256 - Implement export_core() and import_core()

crypto: sha256 - Implement export_core() and import_core()

Since commit 9d7a0ab1c753 ("crypto: ahash - Handle partial blocks in
API"), the recently-added export_core() and import_core() methods in
struct shash_alg have effectively become mandatory (even though it is
not tested or enforced), since legacy drivers that need a fallback
depend on them.  Make crypto/sha256.c compatible with these legacy
drivers by adding export_core() and import_core() methods to it.

Reported-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reported-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Closes: https://lore.kernel.org/r/aLSnCc9Ws5L9y+8X@gcabiddu-mobl.ger.corp.intel.com
Fixes: 07f090959bba ("crypto: sha256 - Use same state format as legacy drivers")
Tested-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Tested-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Link: https://lore.kernel.org/r/20250901165013.48649-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>