feat(boil)!: Output structured data in boil-target-tags file - #1598

Part of https://github.com/stackabletech/internal-issues/issues/169.

Our actions/workflows need to know which tag boil considers the canonical tag in order to point to the correct fully-qualified image tags in the image index manifest.

As such, the boil-target-tags file now contains structured (JSON) data in the following form:

{
  "krb5": {
    "canonical": "localhost/sdp/krb5:1.21.1-stackable26.7.0-amd64",
    "others": [
      "localhost/sdp/krb5:1.21.1-stackable26.7-amd64"
    ]
  },
  "testing-tools": {
    "canonical": "localhost/sdp/testing-tools:0.3.0-stackable26.7.0-amd64",
    "others": [
      "localhost/sdp/testing-tools:0.3.0-stackable26.7-amd64"
    ]
  }
}

boil build krb5=1.21.1 testing-tools=0.3.0 --image-version 26.7.0 --use-localhost-registry --write-image-manifest-uris --floating-tag --dry

Breaking changes

• The default name of the output file produced by the --write-image-manifest-uris argument is now boil-target-tags.json instead of just boil-target-tags.
• The boil-target-tags.json now contains JSON instead of plain text.