In-depth validation of certificates

[lmammino]

Right now the library only validates a certificate based ONLY on the status of the signature. In reality a certificate can be considered invalid even if the signature is validated correctly.

As far as I understand there are several other factors that we should support in terms of validation:

• Signature validity: already supported ✅
• Certificate emission time and expiry time. The DgcContainer struct already collects these timestamps but we offer no easy way to check the current time against them.
• Business Rules (or country-specific rules): see #19 for dedicated issue.

In the context of this issue I think it will important to figure out an ergonomic API that:

1. should make it easy to validate the certificate in one single operation (function call)
2. Return the certificate data (if we can parse that correctly)
3. Return a clear error in case of validation failed (for instance it's very important to distinguish whether a certificate is expired or whether it doesn't satisfy a specific regional rule)

Maybe we could have a dedicated CertificateValidity struct that can contain various fields like this:

pub struct CertificateValidity {
  signature: SignatureValidity,
  time: TimeValidity,
  business_rules: BusinessRulesValidity
}

SignatureValidity, TimeValidity and BusinessRulesValidity could be enums that can encapsulate all the different state of validation that is relevant for them. For instance:

pub enum TimeValidity {
  Valid,
  NotValidYet,
  Expired
}

Finally we could have a is_valid() method on the CertificateValidity struct that simply returns true or false if all the conditions are satisfied or not...