Idea: Restrict number of keys to upload

[burke]

(This idea came out of a call with another team, whose local health authority is requesting this)

Currently we extract all up-to-14 keys from the framework and upload them all.

One health authority wants to have the user upload keys only since N days before symptom onset.

We could implement this by passing a parameter to OTC generation that associates the OTC with a maximum number of keys, then returns that number of keys to the client with their KeyClaimResponse. That maximum would be verified on upload and the client would be trusted to have chosen the correct subset (continuity validations, etc., will still apply).

This is actually partly implemented already by way of the 28-key limit. We could repurpose that to be a 14-key limit that gains an extra "slot" each day until the key is expired, and provide a way to configure the initial 14 downward.