*-template: Cover relative references

[wking]

As discussed here and here. This allows for entries that can move with the base content. For example, you could have a ref-engines object at file:///srv/app/ref-engines.json with content like:

{
  "refEngines": [
    {
      "protocol": "oci-index-template-v1",
      "uri": "index.json"
    }
  ],
  "casEngines": [
    {
      "protocol": "oci-cas-template-v1",
      "uri": "blobs/{algorithm}/{encoded}"
    }
  ]
}

that now expands to file:///srv/app/index.json, file:///srv/app/blobs/sha256/e97…, etc. And those references would still expand correctly if you moved the whole /srv/app directory to /srv/new-app and referenced the ref-engines object from file:///srv/new-app/ref-engines.json.

[wking]

The Go implementation of RFC 3986's relative resolution is ResolveReference.

[xiekeyang]

LGTM