AIP-84 Make dag runs list sortable by run_after

[bbovenzi]

We added run_after to the DagRunResponse object. But we didn't make the dag runs list to be sortable by run_after

[josix]

Hi @bbovenzi, I can work on this issue, could you assign it to me? Thanks!

[bbovenzi]

Sorry, I already started working on this but forgot to tag myself.