feat: [CI-18552]: add Docker Buildx Bake support with multi-registry capabilities

[Ompragash]

Adds opt-in Docker Buildx Bake support for high-level builds and multi-registry pushes, while maintaining full backward compatibility.

Key Features:

• Bake Mode: Activated via PLUGIN_BAKE_FILE - runs docker buildx bake instead of docker buildx build
• Multi-Registry Push: Single bake command pushes to multiple registries in parallel
• Flexible Options: PLUGIN_BAKE_OPTIONS supports targets, --set overrides, and additional flags (semicolon-delimited)
• Enhanced Config: PLUGIN_CONFIG now accepts file paths OR JSON content (uses os.Stat detection)

New Inputs:

• PLUGIN_BAKE_FILE: Path to bake definition file (HCL/JSON/Compose)
• PLUGIN_BAKE_OPTIONS: Semicolon-delimited bake CLI flags and targets

Backward Compatibility:

• ✅ All existing functionality unchanged when PLUGIN_BAKE_FILE not set
• ✅ PLUGIN_CONFIG enhancement maintains JSON content support
• ✅ Bake and push-only modes are mutually exclusive (with validation)

Benefits:

• Simplified Pipelines: Replace multiple build+push steps with single bake step
• Atomic Multi-Registry: All registries get identical image with same digest
• Better Performance: Parallel pushes handled by buildx
• Declarative Config: Build configuration in version-controlled bake files

Usage Example

  settings:
    bake_file: docker-bake.hcl
    config: /path/to/docker-config.json
    bake_options: "web;api;--progress=plain"

Fixes: Enables efficient multi-registry workflows and advanced BuildKit features through bake files.