Reintegrate Power BI plugin into Microsoft Fabric as a submodule

### Description
Following the unification of Microsoft’s data ecosystem, the `plugin-powerbi` should be reintegrated into the `plugin-microsoft-fabric` repository. This will consolidate Microsoft-related integrations and streamline maintenance.

The integration should be handled as a **submodule** to maintain the history of the original repository while providing a seamless developer experience within the Fabric suite.

### Proposed Changes
* **Submodule Addition**: Add [kestra-io/plugin-powerbi](https://github.com/kestra-io/plugin-powerbi) as a submodule under the `-fabric` repository (e.g., in a `plugins/powerbi` directory).
* **Alias Configuration**: Implement a plugin alias so that existing flows using `io.kestra.plugin.powerbi` continue to function without breaking changes, mapping them to the new location within the Fabric plugin.
* **Build Automation**: Update the Gradle/Maven build configuration to include the submodule during the CI/CD process and artifact generation.
* **Documentation**: Update the README to reflect that Power BI tasks are now part of the Microsoft Fabric plugin bundle.

### Reasons for Change
* **Consistency**: Aligns with Microsoft's branding where Power BI is a core pillar of the Fabric platform.
* **Maintenance**: Reduces the overhead of managing separate CI pipelines and release cycles for closely related Microsoft tools.

### Additional Context
* **Original Repository**: `https://kestra.io/plugins/plugin-powerbi`
* **Target Repository**: `plugin-microsoft-fabric`

Start in the plugin-microsoft-fabric repository by inspecting the plugins/powerbi location, the Gradle/Maven build configuration, and the README. Check how the kestra-io/plugin-powerbi repository can be added as a submodule and how the io.kestra.plugin.powerbi alias should map within Fabric. Done means the submodule, alias, CI/CD and artifact generation integration, and README update all work without breaking existing flows.

Written by the indexing model from the issue text.