allow >1 paralellism for pubsub source to increase ingestion performance

[lmatz]

more discussion can be found here: https://risingwave-labs.slack.com/archives/C0606NNR74P/p1714493567060129

In essence, it is hard to make pubsub source exactly-once and other products only provide at-least-once semantics too, so 1 parallelism only does not help anyways.

We should make it parallel.

cc: @fuyufjh

[lmatz]

Is it possible to have a rough ETA? (not necessarily need to be done very urgently)
as we are about to have a call with the user