Add results to custom task support

[SaschaSchwarze0]

When a Shipwright Build runs as part of a larger pipeline, then this pipeline may want to perform further steps after the BuildRun succeeded. For this, it needs access to image details, in particular the digest. We should pass it (maybe together with other image details) as result of the custom task to the pipeline.

Tekton docs: https://tekton.dev/docs/pipelines/customruns/#monitoring-results

[SaschaSchwarze0]

Idea: have four results for the output image and users can pick what they need:

• image: registry.com/namespace/image:v1.1@sha256:...
• image-name: registry.com/namespace/image
• image-tag: v1.1
• image-digest: sha256:...

Name and tag would be used from Build(Run) spec output with tag defaulting to latest. The image-digest would be taken from status output, and would only be set if shipwright pushed the image, or if the build strategy did set the shp-image-digest result.

Based on future Build features, we can extend this with more results such as image-vulnerable.

We can also think about adding source-related results.

[liangyuanpeng]

I'm make a demo of gitops with shipwright and argocd.

In my case, i need this result to update image tag to git repo.

Once I figure out what's going on here I'll work for it.

Pending for #44

/assign