feat: add module backend assignment support

[leejet]

Summary

• Add --backend and --params-backend module assignment support.
  • Supports module assignments such as te=cpu,vae=cuda0,diffusion=vulkan0.
• Add SDBackendManager to own and share backend instances by resolved backend name.
• Pass runtime and parameter backends into GGMLRunner externally.
• Split ggml_extend_backend.hpp into ggml_extend_backend.h/.cpp.
• Document backend selection behavior in docs/backend.md.

Related Issue / Discussion

Since #1184, there have been multiple PRs containing backend selection related changes. However, none of them achieved the behavior I expected, and the implementations were intertwined with other unrelated changes. I decided to write a standalone implementation myself that better matches my design goals, while also adding support for parameter-based backend selection.

This PR was inspired by the work in stduphf’s PR. When merging this PR, I will add @stduhpf as a co-author.

Additional Information

• Ran smoke tests with several model pipelines

Checklist

• I have read and confirmed this PR follows the contribution guidelines.