Enable support for "multi-use" QR codes

[swcurran]

Based on the plan for Safe Entry, we'll have a need for multi-use QR codes that people print and display. The current process (I think) is:

• Generate the proof request
• Store it in a database, returning a UUID for the request
• Display the UUID as part of the QR code
• When the QR code is processed the URL resolves to the UUID and then the proof request
• Send the proof request to the agent

Propose we make two changes:

• Alter the URL resolution process inject the nonce into the proof request during the resolution process, just before sending it to the agent.
  • This should be done for all proof requests, single or multi-use
• Add and flag when creating a proof request, enabling it to be a multi-use request suitable for printing and handle the UUID and proof request records in the database to be handled appropriately.

NOTE: There is a separate issue #61 where I suggest in a note that we eliminate the statefulness of the solution. This feature WOULD require statefullness, as this QR code would have to last across pod reboots. As such, I'll probably remove that comment.