feat: reverse scans public preview

[igorbernstein2]

This adds a reversed boolean to Query, which will allow endusers to stream rows in reverse order.

Example:

Query query = Query.create("alphabet").range("a", "z").limit(3);
ServerStream<Row> results = client.readRows(query);

for (Row row : results) {
  System.out.println(row.getKey().toStringUtf8());
}
// Prints z, y, x

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/bigtable/Reads.java:203, tag bigtable_reverse_scan

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