WIP: PIP 97 Async HTTP Auth Filter - #20309

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Hi @michaeljmarshall, thanks for starting this. It was left as an unfinished draft, and the servlet code has since moved from javax to jakarta, so it would need a fresh start. The HTTP part of PIP-97 is still pending on master, and I've noted that on #12105 so it stays tracked. We're going through the backlog of open PRs (https://lists.apache.org/thread/cgy7w0qgnq8s7j0ykyzgrn3hj4nb67wf), so I'm closing this one. If you'd like to continue, feel free to reopen it rebased on current master.