gh-99108: Implement HACL* HMAC (#130157)

A new extension module, `_hmac`, now exposes the HACL* HMAC (formally verified) implementation.

The HACL* implementation is used as a fallback implementation when the OpenSSL implementation of HMAC
is not available or disabled. For now, only named hash algorithms are recognized and SIMD support provided
by HACL* for the BLAKE2 hash functions is not yet used.