Resolve beam_fn_api flag that changes how side inputs are represented in Beam #20645
Description
Currently the core SDK has a flag beam_fn_api that changes how side inputs are constructed. The difference is minor. The portable expansion already works on all runners except Dataflow and should be the default. This is part of making portability the default generally, at least for the core SDK and model.
Imported from Jira BEAM-11392. Original Jira may contain additional context.
Reported by: kenn.