Provide a stable hash function in safelib
Summary
Some use cases (SAML eduPersonTargetedID, pairwise-id, subject-id, OIDC pairwise subject ID...) require computing an opaque value from a stable attribute
We should provide a function for this that can be used in macros, it should not depend on the key like 'encrypt' does because it must stay stable in long periods of time
Design proposition
Make something like hmac_sha256 available in safejail, if possible