js-legacy: Use devnet deployment of stake pool program (#177)

* js-legacy: Use devnet deployment of stake pool program

#### Problem

There's a new devnet deployment of the stake pool program, but the JS
library can't use it.

#### Summary of changes

Add the new constant, and properly parametrize all types to take in a
program id as applicable.

* Unbreak by making program id optional

Author: joncinque

clients/js-legacy/src/constants.ts

@@ -10,6 +10,11 @@ export const METADATA_MAX_URI_LENGTH = 200;
 // Public key that identifies the SPL Stake Pool program.
 export const STAKE_POOL_PROGRAM_ID = new PublicKey('SPoo1Ku8WFXoNDMHPsrGSTSG1Y47rzgn41SLUNakuHy');
 
+// Public key that identifies the SPL Stake Pool program deployed to devnet.
+export const DEVNET_STAKE_POOL_PROGRAM_ID = new PublicKey(
+  'DPoo15wWDqpPJJtS2MUZ49aRxqz5ZaaJCJP4z8bLuib',
+);
+
 // Maximum number of validators to update during UpdateValidatorListBalance.
 export const MAX_VALIDATORS_TO_UPDATE = 5;