`request_timeout` on Dune sources

[fleupold]

Currently request_timeout is only supported if Dune is the destination of a query. It would be useful to also populate that parameter of source queries that might run complex joins.

[bh2smith]

I took a look into this and it turns out that the AsyncDuneClient class used for DuneSources doesn't expose the configurable request timeout. However, it can be easily added. So this will require a slight modification to the the Dune client library.

Like so duneanalytics/dune-client#152