add fields to encode "expected by" and express time extensions

[rrix]

resolves #37

This is a fairly general model that provides PIPs/CBs the ability to:

• set a "best effort" expectation on request acknowledgement. This can or should be set to the legal regime's timelines if there is no estimate.
• express to End User that a request is going to take longer than the times originally expected or prescribed
• there is a general messaging field called processing_details which will make it feasible for AAs to show a simple status message provided by PIP/CB to extend the general state/reason fields to provide a reason for an extension. This could be useful in terminal states as well, and follows on an implicit point which already existed in the specification

I believe this approach is quite flexible and applicable to many data regimes rather than encoding specific California behaviors.

[rrix]

I'm planning to land this during the week unless anyone has any feedback.