FEAT: public-key endpoint accepts optional date for historical key lookup

[YaroslavVlasenko]

• Adds an optional date (minutes since 2020-01-01 UTC) to OwidController.GetPublicKey, plus a pluggable IPublicKeyStore abstraction.
• Default ConfigurationPublicKeyStore serves the single configured key (date ignored) - existing single-key consumers are unchanged (the controller's store is optional and defaults to it). DatedKeyStore is a reference implementation of the selection rule (newest key with Created <= date).
• 404 when no key is active at the requested date; [ApiController] returns 400 for a malformed date.