Add mailbox version const to SDK (#3125)

.changeset/healthy-waves-cheat.md

@@ -0,0 +1,5 @@
+---
+'@hyperlane-xyz/sdk': patch
+---
+
+Add mailbox version const to SDK

typescript/sdk/src/consts/mailbox.ts

@@ -0,0 +1,2 @@
+// Pairs well with /utils/src/messages.ts#formatMessage
+export const MAILBOX_VERSION = 3;

typescript/sdk/src/index.ts

@@ -29,6 +29,7 @@ export {
   hyperlaneContractAddresses,
   hyperlaneEnvironments,
 } from './consts/environments';
+export { MAILBOX_VERSION } from './consts/mailbox';
 export { defaultMultisigConfigs } from './consts/multisigIsm';
 export { SEALEVEL_SPL_NOOP_ADDRESS } from './consts/sealevel';
 export {