Expose PubsubSource to create UnboundedSource and utilize withMaxNumRecords from BoundedReadFromUnboundedSource

We are ingesting from a Pubsub Read and are attempting to mimic the maxNumRecords that was available in 0.6.0. In order to do this we would need to utilize withMaxNumRecords from the BoundedReadFromUnboundedSource class. We would need to utilize the PubsubSource class to create the UnboundedSource from Pubsub. Would it be possible to expose PubsubSource? Currently what is the recommended way to create a bounded read from Pubsub with a set number of records?

Imported from Jira [BEAM-2715](https://issues.apache.org/jira/browse/BEAM-2715). Original Jira may contain additional context.
Reported by: alevy.

Start by reading the PubsubSource and BoundedReadFromUnboundedSource APIs referenced in the issue, then trace how Pubsub Read currently constructs its source. Determine how PubsubSource could be exposed and used with withMaxNumRecords, and verify the recommended bounded-read behavior with focused tests for a fixed record limit.

Written by the indexing model from the issue text.