Add timeout experimental feature to the spec

[lionelvillard]

Added timeout to the delivery spec. For more details: knative/eventing#5149

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[evankanderson]

I think we may want to have a bit of a level-setting discussion here; the goal of the specs repo is to have a minimal, useful specification for what a conformant implementation looks like, rather than to document every behavior of the current OSS implementation.

I suspect that we may want to remove the "update the spec" suggestion in the eventing template, as the goal is that the Knative spec evolves more slowly than the OSS implementation, which can make experimental and backwards-incompatible changes without requiring a new spec release and vendors to update their software in order to maintain conformance.

[evankanderson]

/hold

[dprotaso]

@pierDipi is this still relevant? /unassign @evankanderson