Add built-in function to get runtime info

[tsandall]

These changes add support for accessing runtime information inside of
policies. In some cases, policies need to access environment variables
or configuration that OPA was booted with. These changes add a built-in
function that allows policies to gain access to this information. The
built-in function itself is relatively trivial. Most of the required
changes were plumbing the runtime information from the entrypoint down
into the evaluation engine. The alternative would have been to introduce
a global variable containing this information however that would be have
been harder to reason about in library integrations.

Fixes #420

Signed-off-by: Torin Sandall [email protected]

[tsandall]

Example:

$ curl 'localhost:8181/v1/query?q=opa.runtime(x)&pretty'
{
  "result": [
    {
      "x": {
        "config": {
          "bundle": {
            "name": "http/example/authz",
            "polling": {
              "max_delay_seconds": 120,
              "min_delay_seconds": 60
            },
            "service": "acmecorp"
          },
          "decision_logs": {
            "reporting": {
              "max_delay_seconds": 600,
              "min_delay_seconds": 300
            },
            "service": "acmecorp"
          },
          "labels": {
            "app": "myapp",
            "environment": "production",
            "region": "west"
          },
          "services": [
            {
              "credentials": {
                "bearer": {
                  "token": "bGFza2RqZmxha3NkamZsa2Fqc2Rsa2ZqYWtsc2RqZmtramRmYWxkc2tm"
                }
              },
              "name": "acmecorp",
              "url": "https://example.com/control-plane-api/v1"
            }
          ],
          "status": {
            "service": "acmecorp"
          }
        },
        "env": {
          "Apple_PubSub_Socket_Render": "/private/tmp/com.apple.launchd.qwJ5XNX4FX/Render",
          "COLORFGBG": "7;0",
          "COLORTERM": "truecolor",
          "COMMAND_MODE": "unix2003",
          "EDITOR": "vim",
          "GOPATH": "/Users/torin/go",
          "HISTCONTROL": "erasedups:ignorespace",
          "HISTSIZE": "99999",
          "HOME": "/Users/torin",
          "ITERM_PROFILE": "Default",
          "LANG": "en_US.UTF-8",
          "LANGUAGE": "en_US.UTF-8",
          "LC_ALL": "en_US.UTF-8",
          "LESS_TERMCAP_mb": "\u001b[1;31m",
          "LESS_TERMCAP_md": "\u001b[1;31m",
          "LESS_TERMCAP_me": "\u001b[0m",
          "LESS_TERMCAP_se": "\u001b[0m",
          "LESS_TERMCAP_ue": "\u001b[0m",
          "LESS_TERMCAP_us": "\u001b[1;32m",
          "LOGNAME": "torin",
          "OLDPWD": "/Users/torin",
          "PWD": "/Users/torin/go/src/github.com/open-policy-agent/opa",
          "SHELL": "/bin/bash",
          "SHLVL": "1",
          "TERM": "xterm-256color",
          "TERM_PROGRAM": "iTerm.app",
          "TERM_PROGRAM_VERSION": "3.2.0",
          "USER": "torin",
          "VIRTUAL_ENV": "/Users/torin/venv",
          "XPC_FLAGS": "0x0",
          "XPC_SERVICE_NAME": "0",
          "_": "./opa_darwin_amd64",
          "__CF_USER_TEXT_ENCODING": "0x1F5:0x0:0x52"
        }
      }
    }
  ]
}