Add keyset expiry field to `/v1/keysets` response

[callebtc]

Keysets should have a pre-announced expiry so that wallets can either rotate to new keysets or recognize that a mint is shutting down.

[a1denvalu3]

Related? davidcaseria#1

[ok300]

Agree, I think this is redundant, as a keyset expiry field is already part of the Keyset ID V2 PR (#182) , thanks to @lollerfirst 's contribution to the PR (linked above).