Require sink at the end of a branch of the pipeline

[fpacifici]

This is an alternative approach to #218.

The rust adapter chains map and filter steps together in one step in rust.
This means that, when a map or filter is processed, it is not added to the
Rust consumer immediately. This only happens when the chain is finalized.

that happens when we encounter a step that cannot be chained or a sink.

Problem: if there is a sequence of map operations and no sink, these were
never added to the rust consumer.

This PR requires aa Sink to the end of each branch.
This idea depends on the assumption that, if you terminate a branch with
anything that can produce output, is likely wrong.
If a step can produce an output and it is at the end of the branch,
what should we do with those messages ?

Requiring a sink implies asking this question to the author of the
pipeline and letting them decide.

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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• Require sink at the end of a branch of the pipeline by fpacifici in #230

• Move gcssink config override out of the adapter by fpacifici in #228

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[evanh]

This would be my preferred way of handling this case. Once we have a sink primitive for tasks, I think that will cover all the cases.