Added possibility to use gcs push endpoint on pubsub subscription

[luigi-bitonti]

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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[juliocc]

thanks @luigi-bitonti! this lgtm.

[luigi-bitonti]

It's a pleasure @juliocc ! Do I have to fmt also the documentation? I see that it didn't pass a check about formattation.

[juliocc]

Yes, just run your code through terraform fmt