V2 API: Expose extraArgs for parity with CLI flags

Understand v2 is unstable and in development - would be helpful to have `extraArgs` (or similar) so we can access most v1 options via CLI flags while the proper v2 interface is being built out.

Low effort, high reward escape hatch that unblocks usage without requiring each option to be explicitly added to `SDKSessionOptions`.

```typescript
// v1 has this
extraArgs?: Record;

// v2 SDKSessionOptions doesn't
```