Add Firestore query tombstone snippets

[sorced-jim]

Description

Add query tombstone snippets to be used https://cloud.google.com/fire…store/docs/best-practices#avoid_skipping_over_deleted_data

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

• firestore/cloud-client/snippets.py:1035, tag firestore_query_with_tombstones
• firestore/cloud-client/snippets.py:1055, tag firestore_query_without_tombstones

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[dandhlee]

Hey @sorced-jim, could you address the lint errors? Give me a ping internally if you can't see the Kokoro logs.