Related work items: microsoft#1416, microsoft#1558, microsoft#1572, microsoft#1597, microsoft#1600, microsoft#1607, microsoft#1611, microsoft#1616

* Adding a simulator + regopolicyinterpreter.

This PR separates all the interaction with Rego into its own extractable package
called `regopolicyinterpreter`. Instead of calling Rego directly,
the `securitypolicy` package now uses this package to implement Rego policies.
Separating out the Rego interpreter behavior in this way allows the same
code to be used by a new `policyenginesimulator` tool, which provides the
ability to simulate security policy execution on the command line.

`regopolicyinterpreter` exposes various Rego things like modules and metadata
in a typed way to make them easier to work with:
    - `RegoPolicyInterpreter` is the main interface
    - `RegoModule` is a standalone Rego module that can be included in the
       policy execution. There are `AddModule` and `RemoveModule` methods for
       modifying the interpreter to include various modules.
    - `RegoQueryResult` wraps the results that come from the Rego policy with
       some useful methods for extracting scalar data types
       (i.e. `bool`/`int`/`float`/`string`)
    - `EnableLogging` provides a way to get multiple levels of policy logging
      for debugging purposes, ranging from `Info`, which will output prints that
      come from the Rego policy itself, to `Metadata`, which will dump the
      entire policy metadata structure to the log with each interaction. This is
      primarily intended for offline use (e.g. by the simulator).

The `policyenginesimulator` tool uses `RegoPolicyInterpreter` to simulate
policy enforcement. Usage:

```
  -commands string
        commands JSON
  -data string
        initial data state
  -log string
        log path
  -logLevel string
        None|Info|Results|Metadata (default "Info")
  -policy string
        policy Rego
```

The commands JSON allows the user to specify the type and order of the commands
send by the host to the guest that will interact with the simulated policy, for
example:

``` json
[
    {
        "name": "load_fragment",
        "input": {
            "issuer": "did:web:contoso.github.io",
            "feed": "contoso.azurecr.io/custom",
            "namespace": "custom",
            "local_path": "custom.rego"
        }
    },
    {
        "name": "mount_device",
        "input": {
            "target": "/mnt/layer0",
            "deviceHash": "16b514057a06ad665f92c02863aca074fd5976c755d26bff16365299169e8415"
        }
    },
    {
        "name": "mount_overlay",
        "input": {
            "target": "/mnt/overlay0",
            "containerID": "container0",
            "layerPaths": [
                "/mnt/layer0"
            ]
        }
    },
    {
        "name": "create_container",
        "input": {
            "containerID": "container0",
            "argList": [
                "/pause"
            ],
            "envList": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "TERM=xterm"
            ],
            "mounts": [],
            "workingDir": "/",
            "sandboxDir": "/sandbox",
            "hugePagesDir": "/hugepages"
        }
    }
]
```

Signed-off-by: Matthew A Johnson <matjoh@microsoft.com>