feat: add key rotation for validators

[juan518munoz]

Summary

Continues the transaction input encryption work from #2342 by adding a key rotation flow for the shared transaction encryption key.

Operators schedule a rotation by restarting every validator with two new options:

1. --encryption-key.next.hex / MIDEN_VALIDATOR_NEXT_ENCRYPTION_KEY with the new shared secret.
2. --encryption-key.next.rotation-block / MIDEN_VALIDATOR_NEXT_ENCRYPTION_KEY_ROTATION_BLOCK with the block at which it takes effect.

Changelog

[[entry]]
scope       = "validator"
impact      = "added"
description = "The shared transaction encryption key can be rotated. Operators schedule a rotation with `--encryption-key.next.hex` and `--encryption-key.next.rotation-block`, and `GetTransactionEncryptionKey` announces the next key ahead of the rotation."

[[entry]]
scope       = "rpc"
impact      = "breaking"
description = "`GetTransactionEncryptionKey` now returns a `TransactionEncryptionKeyResponse` wrapping the current key and, when a rotation is scheduled, the announced next key."