Add Terraform provider management

Summary

Explore and add Terraform provider management support to boilerplates.

Problem / context

Terraform templates often need provider configuration, required provider versions, provider aliases, and possibly shared provider defaults. We should define how the CLI and/or template library should manage this consistently.

Open questions

• Should provider management be handled by the CLI, the Terraform module spec, archetypes, templates, or a combination?
• Should users be able to select providers interactively during generation?
• How should provider versions be pinned and updated?
• How should provider aliases and multi-region/multi-account provider configs be represented?
• Should generated provider files be separate files, injected into existing Terraform files, or controlled by template conventions?

Potential scope

• Define a provider metadata/config model for Terraform templates
• Support common provider blocks and required_providers generation
• Add tests/archetypes once the implementation approach is clear

This issue is intentionally exploratory until we decide the exact implementation design.