feat: [CI-17135]: Add push-only mode and tar file support to Drone Buildx Plugin PR Description

[Ompragash]

This PR enhances the Drone Buildx Plugin by adding three new features:

1. Push-Only Mode (PLUGIN_PUSH_ONLY): Enables pushing existing Docker images without building them:
   - Skips the build process entirely
   - Verifies images exist in the local daemon before pushing
   - Supports multiple tags
   - Preserves existing functionality for artifact files and adaptive cards

2. Source Tar Path (PLUGIN_SOURCE_TAR_PATH): Allows loading Docker images from a tar file before pushing:
   - Used in conjunction with push-only mode
   - Loads images from the specified tar file into the local daemon
   - Provides clear error messages if loading fails

3. Tar Path (PLUGIN_TAR_PATH): Saves built or existing images to a tar file:
   - Works in dry-run mode
   - Automatically enables the --load flag when needed
   - Creates parent directories if they don't exist
   - Verifies images exist before attempting to save

Testing:

• Push-only mode works correctly with existing images LINK
• Images can be loaded from tar files and then pushed LINK
• Built images can be saved to tar files in dry-run mode LINK
• Push-only with multiple tags LINK
• All existing functionality continues to work as expected