Support the timestamp assertion as documented in the 2.2 spec. This will likely be automatically generated by the SDK since we already have the ability to call timestamp services. I'd expect it to be a setting for builder to enable this, rather than an assertion users write, but adding one manually should be supported. On the reading side, there are some open questions about how to integrate this into existing signature reports or deal with it as as separate assertion. We will need to figure out a standard way to handle these timestamp updates in the UI.
Validation will need to respect the existence of a valid timestamp assertion.
This assertion will generally be added as an update assertion to that needs to be tested.