lib/crypto: sha256: Add HMAC-SHA224 and HMAC-SHA256 support

Since HMAC support is commonly needed and is fairly simple, include it
as a first-class citizen of the SHA-256 library.

The API supports both incremental and one-shot computation, and either
preparing the key ahead of time or just using a raw key.  The
implementation is much more streamlined than crypto/hmac.c.

I've kept it consistent with the HMAC-SHA384 and HMAC-SHA512 code as
much as possible.

Testing of these functions will be via sha224_kunit and sha256_kunit,
added by a later commit.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160645.3198-9-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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