feat(secret): wire SecretProtocol to irpc with SecretServiceActor

Apply #[rpc_requests(message = SecretMessage)] to SecretProtocol enum with
#[rpc(tx=oneshot::Sender<Result<T, SecretServiceError>>)] and #[wrap] attributes
on each variant. Add SecretServiceActor that wraps SecretServiceHandle and
processes SecretMessage variants via mpsc channel. Update DerivedKey
serialization to use is_human_readable() so postcard preserves private_key
bytes while JSON redacts them. Add Serialize/Deserialize to SecretServiceError
for irpc wire format compatibility. Add tokio dependency for actor runtime.

crates/alknet-secret/Cargo.toml

@@ -27,6 +27,7 @@ rand = "0.8"
 base64 = "0.22"
 irpc = { workspace = true }
 irpc-derive = { workspace = true }
+tokio = { version = "1", features = ["sync", "rt", "macros"] }
 secp256k1 = { version = "0.29", optional = true }
 
 [dev-dependencies]