BarrageStreamGeneratorImpl#getSubView Should Be Bifurcated into FullSubscriptionView and ViewportSubscriptionView

[nbauernfeind]

While reviewing #6347 it became clear how much simpler it would be to review this particular piece of code if the interface was split into at least two view-like implementations, possibly three.

• Full Subscriptions should have their own entry point.
• Viewport Subscriptions should have their own entry point.
• Possibly Growing Viewports should have their own entry point.

We also need a design document, deephaven.io level documentation, or perhaps just very good javadoc in this file, describing the three strategies and how one can expect the RecordBatch to be structured and populated.