admin: apply flow control via high/low watermarks

[jmarantz]

Commit Message: This builds on #19831 , adding an attempt to use http filter methods to deploy flow-control, so we can pause generation of admin output until clients are ready for it.

This is not ready yet -- @alyssawilk has explained to me how to get the flow-control to work

• use high-watermark callback to set a bit in the filter to avoid sending more data
• use low-watermark callback to clear the above bit and send another chunk, and then queue up a callback for the next chunk (assuming high-water callback is not called).

This pattern should also be in the http cache filter and also the Router, though the latter is a lot more complex.
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Caused by: #31087 was opened by jmarantz.

see: more, trace.

[jmarantz]

This PR replaces #19898 which got too stale.

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 7 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[github-actions]

This pull request has been automatically closed because it has not had activity in the last 37 days. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!