Crypto Agility in Shared State Retry

[martinduke]

Secure ciphers are much more important for Shared State Retry than for the load balancer stuff and Initial Encryption. Therefore, some crypto agility to allow this mechanism to adapt as ciphers deprecate would be good.

The key sequence can also imply a cipher, so we'll add that to the data model. Some fields will vary length based on the cipher.

We should also make it so everyone MUST support AES128-GCM.