[RFC-0012] Implement ExternalArtifact API and SDK

[stefanprodan]

This issue tracks the implementation of RFC-0012.

API Tasks:

• Extract the Artifact type from source-controller to github.com/fluxcd/pkg/apis/meta package
• Refactor source-controller to use the Artifact type from the meta package
• Add the ExternalArtifact API to the source-controller API package
• Add documentation for ExternalArtifact to source-controller API docs
• Add the ExternalArtifact API as a supported sourceRef in kustomize-controller including watchers
• Add the ExternalArtifact feature gate to kustomize-controller
• Add the ExternalArtifact as a supported chartRef in helm-controller including watchers
• Add the ExternalArtifact feature gate to helm-controller
• Add documentation for the feature gate and supported sources in kustomize-controller and helm-controller

SDK Tasks:

• Extract the source-controller storage component to a dedicated package
• Refactor source-controller to use the github.com/fluxcd/pkg/artifact package
• Define the public interfaces for the SDK
• Implement the server, client and storage components
• Add development guide for the SDK to the GitOps Toolkit documentation

Artifact Generators Tasks:

• Implement artifact generators in source-watcher controller using the ExternalArtifact API and SDK
• Develop conformance testing for ExternalArtifact generators and consumers

CLI Tasks:

• Add support for ExternalArtifact source type to the following commands:
  • flux trace (display ExternalArtifact.spec.sourceRef)
  • flux reconcile ks/hr --with-source (skip external artifact annotation)
• Add command flux get source external
• Add command flux export source external

[dgunzy]

@stefanprodan if there is a small part of this I can take on I would be keen to help 😃