Remove the need to patch up the coder for DataflowRunner iterable side inputs on the unified worker

FnApiRunnerTest.test_flattened_side_input failed without the fixup because the proto and JSON descriptions differed. One was "keyed" and the other was not causing failures during transcoding from the materialized form to the Fn API compatible form.

Imported from Jira [BEAM-9173](https://issues.apache.org/jira/browse/BEAM-9173). Original Jira may contain additional context.
Reported by: lcwik.

Start with FnApiRunnerTest.test_flattened_side_input and reproduce the failure without the coder fixup. Trace how the materialized form is transcoded into the Fn API-compatible form, comparing the proto and JSON descriptions, and consider the issue done when the test passes without the patch-up and both descriptions agree about whether the input is keyed.

Written by the indexing model from the issue text.