Implement standard device key format

[clintfred]

We are going to standardize the DeviceContext/DeviceKeys JSON format across all SDKs. The proposed format (in Typescript) is:

{
    deviceId: number;
    accountId: string;
    segmentId: number;
    devicePrivateKey: Base64String;
    // “expanded private key” (both pub/priv)
    signingPrivateKey: Base64String; 
}

see: IronCoreLabs/ironoxide#25

[ernieturner]

Updated to add deviceId to this response structure as well.