Implement standard device key format

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: https://github.com/IronCoreLabs/ironoxide/issues/25

Start by reviewing the proposed TypeScript format in this issue and the referenced issue 25, then locate the DeviceContext and DeviceKeys definitions across the Node.js SDK. Done means the standardized JSON shape is implemented consistently, including the listed identifiers and key fields, with validation or tests confirming the format.

Written by the indexing model from the issue text.