chore(docs): include imports in snippets

[daniel-sanche]

This PR moves imports into each region tag (where missing), to make copy/pasting easier

internal bug: 372733665

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/filters/filter_snippets.py:335, tag bigtable_filters_print
• samples/snippets/reads/read_snippets.py:147, tag bigtable_reads_print

You are about to delete 2 region tags.

• samples/snippets/filters/filter_snippets.py:16, tag bigtable_filters_print
• samples/snippets/reads/read_snippets.py:16, tag bigtable_reads_print

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