Store and return testDate on verify code API

[mikehelmick]

TL;DR

Mobile apps would like to differentiate between testDate and symptomDate for purposes of prompting the user.

Currently this is collapsed to a single date that is stored and transported through the verify response, verification token, and verification certificate.

Design

Proposal

1. add TestDate to the verification code struct
2. store this in the DB
3. when the verificaiton token is minted, expand the subject to contain diagnosis.symptomdate.testdate
4. verification token structure will remain the same - a single date called symptom date, that can be either.

/kind enhancement

[mikehelmick]

/assign