Add json/yaml output to flux push artifact

[stefanprodan]

This PR enables extracting the OCI artifact image name and digest from the flux push artifact output in CI.

Example output:

$ .flux push artifact oci://localhost:5050/my-app-config:v1.0.0 --source=local --revision=local --path ./manifests/ -ojson
{
  "url": "oci://localhost:5050/my-app-config@sha256:ccbcfcf35161d3b8127a9658c0dcda9a69ec099d22031269930b4e0e3038779c",
  "repository": "localhost:5050/my-app-config",
  "tag": "v1.0.0",
  "digest": "sha256:ccbcfcf35161d3b8127a9658c0dcda9a69ec099d22031269930b4e0e3038779c"
}

Example of signing the pushed artifact with cosign:

  digest_url = $(flux push artifact  \
	oci://ghcr.io/org/config/app:$(git rev-parse --short HEAD) \
	--source="$(git config --get remote.origin.url)" \
	--revision="$(git branch --show-current)/$(git rev-parse HEAD)" \
	--path="./path/to/local/manifest.yaml" \
	--output json| \
	jq -r '. | .repository + "@" + .digest')
  cosign sign $digest_url

[hiddeco]

Looking great, thanks @stefanprodan 💯 🙇

[seh]

Thank you.

I wound up going with the following to capture the OCI repository and artifact digest into separate shell variables that I glue together later with @ for feeding into cosign sign:

flux push push artifact \
  # ... \
  --output=json |
jq --raw-output '[.repository, .digest] | @tsv' |
read repository digest