Support multiple ports in a revision

## In what area(s)?

/area API
> /area autoscale
> /area build
> /area monitoring
> /area networking
> /area test-and-release

Other classifications:
> /kind good-first-issue
> /kind process

/kind spec

## Describe the feature

A developer using Knative Serving when creating applications that consist of one or more containers should be able to specify the desired port to be managed by Knative and not to be restricted to: one port per serving container and no ports for the other not serving containers eg. sidecars. Note here that there is no intention to add support for multiple-ports managed by Knative but users should be able to specify other ports freely.This has been discussed previously [here](https://github.com/knative/serving/issues/7140) and there were requests [here](https://github.com/knative/serving/issues/6792) and [here](https://github.com/knative/serving/issues/8047).

I have started a proposal [here](https://docs.google.com/document/d/1G-RF0RIhC-leJZHKZJKcB2lKiJnKyaiRPmDtGlC5ouQ/edit?usp=sharing)

Start with the proposal and the prior discussions in issues 7140, 6792, and 8047 to establish the intended API and port-selection behavior. Done means the agreed specification clearly allows additional container ports while identifying which port Knative manages.

Written by the indexing model from the issue text.