Make the specification of an output image optional

[gabemontero]

Although somewhat analogous to optional image push step there is also the concept of allowing the Build output image to be an optional field.

Reminder, it is already optional in the BuildRun, albeit that is simply because the user might not want to override what is specified in the Build.

Discussed in the Nov 8 2021 community meeting, this came up recently in a discussion in @sbose78 's image signature prototype, where I noted that the output image is optional with OpenShift Builds, and cited some of the reasons as to why which are relevant to Shipwright, most notably that the desired output of the build could something other than, or in addition to, a container image.

We also discussed some testing scenarios which warrant not wanting to require definition of an output image.

@adambkaplan this could also fall under the recent openshift build compatibility thread you are having with Siamak