Refactors the previously added DataConnect CRUD helper methods (`insert`, `insertMany`, `upsert`, `upsertMany`) based on feedback.

The core implementation logic (input validation, data serialization using `objectToString`, GraphQL mutation string construction, and calling `executeGraphql`) has been moved from the `DataConnect` class (`data-connect.ts`) to the `DataConnectApiClient` class (`data-connect-api-client-internal.ts`).

The methods in the `DataConnect` class now act as simple pass-through delegates to the corresponding methods on the internal client instance.

Unit tests have been updated accordingly:
- Tests in `index.spec.ts` now verify the delegation from `DataConnect` to `DataConnectApiClient`.
- New tests have been added to `data-connect-api-client-internal.spec.ts` to cover the implementation details within `DataConnectApiClient`.

This change improves separation of concerns, keeping the public API surface (`DataConnect`) clean and concentrating the implementation details within the internal client.