Add ARM64 + AMD64 MultiArch build

[MrTolerant]

Currently, the ArgoCD Agent and Principal images are built only for AMD64 architecture.
Please add support for multi-architecture (ARM64 + AMD64) builds to ensure compatibility with heterogeneous clusters and modern ARM-based cloud environments (e.g., Graviton, Apple Silicon).

Proposed solution:

Update the build pipeline to produce and push multi-arch Docker images.

Ensure manifest-list creation for automatic architecture resolution.

Validate builds on both ARM64 and AMD64 environments.

Benefits:

Enables usage in mixed-architecture Kubernetes clusters.

Improves support for ARM-based cloud nodes and local development setups.

Example:

docker buildx build --platform linux/amd64,linux/arm64 -t argocd-agent:multiarch