docs(samples): add bigtable filter snippet

[Sita04]

1. Add a filter snippet to read row using a filter
2. Add region tags to two existing snippets

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/src/main/java/com/example/bigtable/Filters.java:73, tag bigtable_hw_create_filter
• samples/snippets/src/main/java/com/example/bigtable/Filters.java:362, tag bigtable_hw_get_with_filter
• samples/snippets/src/main/java/com/example/bigtable/Filters.java:380, tag bigtable_hw_scan_with_filter
• samples/snippets/src/main/java/com/example/bigtable/Filters.java:400, tag bigtable_print_row

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