[Prism] Support BundleFinalization DoFn parameter

[damondouglas]

This PR closes #31912 by adding support for bundle finalization via the beam.BundleFinalization DoFn parameter.

With Prism running, to validate Java tests against Prism runner:

TEST=org.apache.beam.sdk.transforms.SplittableDoFnTest
./gradlew :runners:portability:java:ulrLoopbackValidatesRunnerTests -PjobEndpoint=localhost:8073 --tests="$TEST"

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[damondouglas]

R: @lostluck

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[lostluck]

General comments since I know you have other plans for that remaining test right now.

[lostluck]

Minor changes and refactoring suggestions!

Please feel free to merge after you've made them, if you don't want another look.

[lostluck]

Please put the standard library imports in a separate group at the top of the imports. For some reason the break was dropped, and the grouping changed.

[lostluck]

Consider instead having the scope passed into this function instead of returning a Pipeline.

eg.

func ParDoProcessElementBundleFinalizer(s beam.Scope)  {
    imp := beam.Impulse(s)
	beam.ParDo0(s, &processElemBundleFinalizer{}, imp)
}

This cuts the boiler plate.

That makes it easier to integrated with the Prism Test suites, and thus ensure we get explicit code coverage over the prism you've written code.

eg. That's how we do the (increasingly mis-named) unimplemented_test.go file.

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/prism/internal/unimplemented_test.go#L74

So you could add the "Finalization" tests to the TestImplemented suite.
DO NOT refactor that test file to fix it, that's an orthogonal change that should be made separately.

[lostluck]

Naturally, the same for the other test cases in this file.