fix(sdk-core): change expireAt for btc staking to string

Ticket: SC-638

TICKET: SC-638

Author: ewangbitgo

modules/sdk-core/src/bitgo/staking/iStakingWallet.ts

@@ -109,7 +109,7 @@ export interface StakeOptions {
   /**
    * btc staking expire time
    */
-  expireAt?: number;
+  expireAt?: string;
 
   /**
    * btc staking reward address