Enable caching of side-input dependent computations

[kennknowles]

Sometimes the kind of computations one wants to perform in startBundle depend on side inputs (and, implicitly, the window). For example, one might want to initialize a (non-serializable) stateful object. In particular, this leads to users incorrectly (in the case of triggered or non-globally-windowed side inputs) memoizing this computation in the first processElement call.

One option would be to fold this into a customizable ViewFn.

Imported from Jira BEAM-1002. Original Jira may contain additional context.
Reported by: robertwb.