oci_discovery/fetch_json: Return the final URI along with the JSON

[wking]

From here:

Note that if the retrieval was the result of a redirected request, the last URI used (i.e., the URI that resulted in the actual retrieval of the representation) is the base URI.

This fills in remaining Python functionality left off from #21. Ref-engine discovery and ref-engine APIs now also return dicts that include a uri key representing this URI, which allows their consumers to also perform reference resolution. For example, tools consuming the Merkle roots may need the URI from which those roots were extracted in order to resolve a CAS-engine URI reference.

[xiekeyang]

Here needn't protocol entry? Maybe both uri and protocol are all necessary.

[wking]

Here needn't protocol entry?

This isn't an engine config, it's the location from which this root was extracted. It would be passed into a subsequent CAS-engine initialization as base, although in this case

.["example.com/app#1.0"].roots[0].root.casEngines[0].uri is a full URI (not a reference), so the base URI won't be needed. It helps with things like:

{
  "example.com/app#1.0": {
    "roots": [
      {
        "root": {
          ...,
          "casEngines": [
            {
              "protocol": "oci-cas-template-v1",
              "uri": "../cas/{algorithm}/{encoded:2}/{encoded}"
            }
          ]
        },
        "uri": "http://example.com/oci-index/app"
      }
    ]
  }
}

So I don't think we need protocol here; just the base URI(s) for resolving any references in the returned root(s).

[xiekeyang]

LGTM