Add Spin (WebAssembly) build backend support

[kingdonb]

Goal: Add Spin (WebAssembly) as a build backend option alongside Kaniko

Background:
Spin provides fast, secure WebAssembly builds that can be orchestrated with SpinKube. This represents the modern evolution of container deployment - faster startup, better security model, and excellent developer experience.

Technical Requirements:

Build Configuration:

# build-config.yaml
build_backend: spin
spin_config:
  template: http-rust  # or http-go, http-js, etc.
  component_name: my-app
platforms:
  - wasm32-wasi
additional_tags:
  - latest
  - v1.0.0

Pipeline Integration:

• Spin build stage: spin build equivalent to Kaniko execution
• Registry push: Push to OCI registry as WebAssembly component
• Multi-arch support: WASI target architecture handling
• Manifest creation: WebAssembly component manifests

Spin-Specific Features:

• Template detection: Auto-detect Spin templates (http-rust, http-go, etc.)
• Component builds: Support multi-component Spin applications
• Local testing: spin up equivalent in pipeline for validation

Integration Points:

• SpinKube deployment: Future integration with Kubernetes SpinKube operator
• Registry compatibility: Ensure WASM components work with standard registries
• Security model: Leverage Spin's capability-based security

Success Criteria:

• Can build Spin applications from Git repositories
• Multi-architecture WASM component support
• Registry push/pull of WebAssembly components
• Clear migration path from container to WASM deployment

References:

• Spin documentation: https://developer.fermyon.com/spin
• SpinKube: https://spinkube.dev/

Priority: Medium (new capability, doesn't block existing features)