Add a default `programId` to ts client

[metaproph3t]

Caveat emptor, this is a non-backwards-compatible change. But I think it makes the interface a little simpler, since you don't have to dig up the OpenBook program ID.

Alternatively, we could put programId after opts, if we think that there's a higher probability of someone changing the opts than the programId

[binyebarwe]

Please request me as a reviewer next time once the PR is ready :)

[metaproph3t]

Will do!