feat: add experimental support for 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

This its currently blocked on generated protos which will be autogenerated soon

[mutianf]

startClosed and end closed is the same key, maybe change to different keys? So customers can know if start key should be < end key or > end key because it's reversed?

[mutianf]

Or maybe in the javadoc example in Query.java update it to:

   * <pre>{@code
   * query
   *   .range(ByteStringRange.unbounded().startKeyClosed("a").endOpen("c"))
   *   .limit(10)
   *   .reversed(true)
   * }</pre>

[igorbernstein2]

I updated the sample to use different keys

[igorbernstein2]

(in #1711)

[igorbernstein2]

closing in favor of #1711 and will address feedback there