docs(samples): Add Dataflow to Pub/Sub snippet

[VeronicaWasson]

Description

Adds a snippet that shows how to write messages to Pub/Sub from Dataflow.

Bug: b/310239684
Doc bug: b/318728052 (Parent doc bug for context: b/309130866)

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• dataflow/snippets/write_pubsub.py:16, tag dataflow_pubsub_write_with_attributes

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[davidcavazos]

For new samples we recommend using type hints, they make the code more self-documented and easier to understand.

Type hints can sometimes be a little tricky to figure out, so feel free to ask if you need any help with any of them.

I believe you can run the type checks with nox -s lint (more info).

[VeronicaWasson]

thanks -- I added type hints. I'm not super familiar with them, so let me know if anything looks wonky!