config: add extensible "Extra" field, use it ActiveConfig

[srenatus]

Previously, extra keys in the OPA config file would be ignored in the /v1/config API, but included when evaluating opa.runtime().config.

With this change, they'll be collected into the Extra field, and included in the response of an API call to /v1/config, too.

👣 Trying out the change

Put this file on disk,

# foo.yml
foo: bar
baz:
  quz: 100
decision_logger:
  console: true

and start OPA via opa run -a 127.0.0.1:8181 -c foo.yml -s.

OPA 0.53.1

$ curl "http://127.0.0.1:8181/v1/config?pretty"            
{
  "result": {
    "default_authorization_decision": "/system/authz/allow",
    "default_decision": "/system/main",
    "labels": {
      "id": "617ef51a-5f78-42ec-ba04-d7ca758c307f",
      "version": "0.53.1"
    }
  }
}
$ curl "http://127.0.0.1:8181/v1/query?pretty" -d '{"query": "x = opa.runtime().config"}'
{
  "result": [
    {
      "x": {
        "baz": {
          "quz": 100
        },
        "decision_logger": {
          "console": true
        },
        "foo": "bar"
      }
    }
  ]
}

This PR

$ curl "http://127.0.0.1:8181/v1/config?pretty"
{
  "result": {
    "baz": {
      "quz": 100
    },
    "decision_logger": {
      "console": true
    },
    "default_authorization_decision": "/system/authz/allow",
    "default_decision": "/system/main",
    "foo": "bar",
    "labels": {
      "id": "4f8394a8-2157-4d5e-a000-07eb6b7abe19",
      "version": "0.54.0-dev"
    }
  }
}
$ curl "http://127.0.0.1:8181/v1/query?pretty" -d '{"query": "x = opa.runtime().config"}'
{
  "result": [
    {
      "x": {
        "baz": {
          "quz": 100
        },
        "decision_logger": {
          "console": true
        },
        "foo": "bar"
      }
    }
  ]
}

[anderseknert]

Looks good to me!