Feature request

Consider adding a new Ksvc flag to specify activation scale value. That represents number of replicas to be scaled from zero when service is activated.

Ref: knative/serving#13161 (note it's an update PR to introduce new & current name of option)
knative/serving#13136 (first PR with former name)

UI Example

kn service create hello --image .../myapp --scale-activation 2

/kind good-first-issue

/cc @dprotaso @psschwei