[Chore] Migrate monorepo configuration to native workspaces

[fderuiter]

Context

The repository currently uses a custom workspace.toml file to manage the core, providers-airflow, and plugins-workflows packages. This is not standard for Python, meaning CI/CD tools, dependabot, and newer resolvers may misinterpret the repository structure.

Implementation Plan

• Deprecate the custom workspace.toml.
• Transition to the standard Poetry 2.0 workspace definition within a single root pyproject.toml, or utilize standard path dependencies (develop = true).

Acceptance Criteria

• workspace.toml is deleted.
• A root pyproject.toml correctly defines the workspace members.
• All cross-package dependencies resolve successfully in local development and GitHub Actions.