[prism] Triggers and Panes and Windowing Strategies

[lostluck]

Triggers are the "last" complex bit of modeling necessary for Prism's Table stakes for being incorporated for use by other SDKs.

Triggers are a way to refine how and when aggregations occur, but require sophisticated accounting Runner side to implement. https://beam.apache.org/documentation/programming-guide/#triggers has the user side details.

Triggers (and WindowingStrategy in general) contribute to the "fun" bit of the batch and stream unification that the Beam model provides. To help manage this, Panes are metadata passed along with the elements, and indicate the upstream aggregation conditions that produced the values.

https://docs.google.com/document/d/12r7frmxNickxB5tbpuEh_n35_IJeVZn1peOrBrhhP6Y/edit describes how Lateness and late data is handled WRT panes.

Ultimately, in prism, this will probably be handled via the state and timer system instead of an additional layer of separate logic, which has the cost of less efficient behavior for "only watermark" triggered aggregations. Robust State and Timers allow for all these.