Add key rotation

[nventuro]

The key registry and getters from #7523 supports key rotation, but the rest of the stack does not. We are not planning on implementing this yet, but can track some of the missing pieces here to later tackle it (if we ever do).

PXE

• needs to be able to handle multiple sets of private keys, and know their different expiration dates.
• needs to perform trial decryption with multiple sets of keys (can choose the correct ones for each block depending on expiration dates)
• needs to be able to respond to npk challenges from the private kernel for all of the historical npks
• needs to be able to decipher the npk used in a given note by trial hashing with the valid npks for a given block (depending on expiration dates)

[nventuro]

We're dropping key rotation as of #8613, so we don't need to do any of this.