Support reverse iteration on pagination

[fedekunze]

Summary

Add support for the ReverseIterator on Paginate and FilteredPaginate queries.

Problem Definition

Some iteration use cases (eg when iterating by block height from latest to earliest) require the reverse iteration in desc order.

Proposal

Add a boolean argument to both functions to support reverse iteration

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[aaronc]

Great. We can also add the boolean to the protobuf pagination message.