Publish install manifests to GHCR and DockerHub as OCI artifacts

Add workflow to build and push the install manifests to:
- ghcr.io/fluxcd/flux-manifests
- docker.io/fluxcd/flux-manifests
The OCI artifacts are signed with Cosign and GitHub OIDC (keyless).
The manifests pushed to GHCR have the container images set to ghcr.io/fluxcd/<controller-name> while the manifests pushed to DockerHub have the controller images set to docker.io/fluxcd/<controller-name>.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>