DUMONT documentation

[kflynn]

This... uh... psuedo-GEP... is an initial spec definition for the ext-authz protocol, with a working name of DUMONT.

I'm obviously abusing the GEP concept a bit here, so to be clear: I do not think that this standard really belongs in the Gateway API docs. It's just a low-friction way for me to let other people see a doc that's previously only existed on my laptop, and it supports Mermaid. Once we figure out a proper home for it, it'll be moved. (I'm not a huge fan of moving it into the XDS repo since, as @htuch notes, it's not part of XDS -- but he's absolutely correct that the XDS repo would be a better place than the Envoy repo. So let's start thinking about it.)

/kind documentation

NONE

(Bonus points to folks who know why it's DUMONT... but not many, it's not a tough reference. 😉)

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[kflynn]

/hold

since I don't want this accidentally merged, either.