Add support for rulesets (business rules) for additional validation of certificates

[lmammino]

Every country publishes country-specific rulesets (also called business rules) that are used to determine if a certificate is valid on a given country based on national rules.

E.g. It's likely that in Italy "negative test certificates" won't be considered valid for most activities.

These rulesets are somewhat documented here: https://github.com/ehn-dcc-development/dgc-business-rules

For Italy you can find the current ruleset at the following URL: https://get.dgc.gov.it/v1/dgc/settings

Interesting enough there is even a black list encode as a rule!