MINOR: jws: introduce jws_b64_hmac_signature() function for HMAC signing

MINOR: jws: introduce jws_b64_hmac_signature() function for HMAC signing

New jws_b64_hmac_signature() duplicates the same functionality as
jws_b64_signature(), but for the use case of HMAC signing.
Intended to be used for ACME EAB.

OpenSSL allows to use EVP_PKEY for HMAC functionality, so
jws_b64_signature() could be reused, but the problem is that although
isn't deprecated it was removed in BoringSSL, and was removed
(due to BoringSSL roots) but then readded back in AWS-LC, because of
"legacy clients" (citing them), for that reason alone I say that having
a dedicated function for hmac is better, HMAC() macro seems to be widely
supported unlike other ways of doing same thing. Another alternative
would be to use EVP_MD API, but it was introduced in OpenSSL 3.0,
so not as widely supported.