Implement GenericClientConfig

[aidenkeating]

@maleck13 Would you mind taking a look? This changes the response from the SDK config endpoint a bit from:

{
    "name": "servicename",
    "config": { "uri": "https://servicename" }
}

to

{
    "name": "servicename",
    "config": {
        "params": { "uri": "https://servicename" },
        "headers": { "app_key": "test", "app_id": "test" }
    }
}

So we'll need to update the sample app, but not the core SDK.