Make 3 unneeded PODTicket fields optional

[artwyman]

As discussed on Telegram, this removes 3 fields from PODTickets in order to get the size of Devcon tickets down to 16 entries. This makes them fit in circuits with a Merkle Depth 5, which is what we optimized for when creating our circuit family for Devcon.

The relevant fields (isRevoked, ticketCategory, timestampConsumed) are unused for Devcon and foreseen use cases, but have been present in tickets for a long time. The approach here is to make them optional at the PODTicket level, and avoid setting them in PODBox pipelines (PretixPipeline, CSVPipeline). This should be relatively safe, but could still break assumptions we're not aware of.

I haven't directly confirmed the new size of Devcon tickets, and the circuits used for ticket proofs. We should test that on staging before going forward.

For this change to be effective we need to push it to Zupass and also to the Devcon PODBox. I'm on the fence about whether or not to merge this, because of the risks. Here are some thoughts on the tradeoffs:

Downside of not merging:

• Ticket proofs will be slower than they should be (probably by 1-4s, and download a larger artifact than they need (roughly 25MB instead of 12MB).

Downside of merging:

• Rob thinks some apps are using query code which looks for all
• Unknown risks of things going wrong because of something we missed.

[artwyman]

I'll let @robknight and @rrrliu decide the timing of merging this, after the parcnet-js SDK is updated with various app dev teams as per proofcarryingdata/parcnet-client#30

[artwyman]

Note that this branch is live on staging-andrew.zupass.org, which also has test Devcon tickets set up on PODBox if you want to use it to test.

[artwyman]

Call is to hold off on this for risk reasons for now. All the performance-sensitive apps have already switched to direct ticket access rather than ZK proofs.
I'm going to keep the PR around in case of need. Will clean up after Devcon.