Support Managed Identity for Application Insights in Data API Builder

**Description:**

Currently, Data API Builder does not support using **Managed Identity** for authenticating with **Application Insights**, which introduces security concerns for production workloads. Relying solely on connection strings without AAD support can be considered insecure.

### Proposal

Implement support for using **Managed Identity** when configuring Application Insights, following the same pattern used in **Azure Functions**:

#### Example:

Set the `APPLICATIONINSIGHTS_AUTHENTICATION_STRING` environment variable with either:

- `Authorization=AAD` for system-assigned managed identity
- `Authorization=AAD;ClientId=` for user-assigned identity

This pattern is already well-known in the .NET ecosystem and avoids introducing another authentication standard.

### Implementation Suggestions

- Add a new property to the **configuration JSON schema** and **CLI tooling** to support setting the `APPLICATIONINSIGHTS_AUTHENTICATION_STRING`.
- Ensure proper **unit test coverage** for this configuration.
- Update documentation accordingly to reflect the supported authentication modes for Application Insights.

### Why This Matters

- Aligning with established Azure/.NET standards reduces learning curves for developers.
- Avoids fragmentation or confusion caused by introducing new or inconsistent patterns.
- Enables secure, keyless telemetry integration using Entra ID (formerly Azure AD).

### Related Work

There is an existing issue for supporting a **Log Analytics sink** which mentions **Entra ID** support ([#2709](https://github.com/Azure/data-api-builder/issues/2709)), but:
- It appears unrelated to **Application Insights**.
- It uses a different configuration syntax that doesn’t align with Application Insights’ authentication mechanisms.

I'll need to know if this feature would be accepted by the team before I start working on it.

Beginne damit, das JSON-Konfigurationsschema und die CLI-Tools zu finden, die die Application Insights-Einstellungen definieren. Prüfe die vorhandenen Unit-Tests für die Konfiguration und den Dokumentationsabschnitt zur Application Insights-Authentifizierung und bestätige anschließend mit dem Team die akzeptierten Authentifizierungsmodi und die erwartete Konfigurationssyntax. Die Aufgabe ist erledigt, wenn Schema, CLI, Tests und Dokumentation die Authentifizierung über managed identity konsistent abdecken.

Vom Indexierungsmodell aus dem Issue-Text verfasst.