control/controlknobs: make Knobs.AsDebugJSON automatic, not require maintenance

The AsDebugJSON method (used only for a LocalAPI debug call) always
needed to be updated whenever a new controlknob was added. We had a
test for it, which was nice, but it was a tedious step we don't need
to do. Use reflect instead.

Updates #14788

Change-Id: If59cd776920f3ce7c748f86ed2eddd9323039a0b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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