Publish SBOM for Flux and the GitOps Toolkit components

[stefanprodan]

We should generate and publish a Software Bill of Materials (SBOM) as a release artifact for all Flux projects. To generate a SBOM for Flux Go modules and their dependencies, we can use Syft and GoReleaser. The published SBOM should be in SPDX format.

Projects:

• flux2
• source-controller
• kustomize-controller
• helm-controller
• notification-controller
• image-reflector-controller
• image-automation-controller
• source-watcher