swarm-secret-client: bound vault requests with a 30s timeout

vaultrs sets no request timeout by default, so a store that accepts a connection and never answers hung every SecretStore call forever. connect_with_identity now sets VAULT_REQUEST_TIMEOUT; a stalled store surfaces as Error::Vault.

swarm-secret-client/src/lib.rs

@@ -74,8 +74,8 @@ pub enum Error {
     #[error("the store's settings are unusable: {0}")]
     Settings(String),
 
-    /// The store refused us, was unreachable, or answered something we could
-    /// not parse.
+    /// The store refused us, was unreachable, did not answer within the
+    /// client's request timeout, or answered something we could not parse.
     #[error(transparent)]
     Vault(#[from] Box<vaultrs::error::ClientError>),