generate operator manifests yaml without kubectl directly

[TLINDEN]

Currently external/svc-postgres-operator.yaml is being generated by downloading the manifests, applying them to a k8s cluster in dry-run mode and then redirecting it to the yaml file.

For one, this leads to last configuration annotation artifacts in the yaml file and it requires the need of a k8s cluster (kind or whatever).

This modification adds a script to do this without k8s. It needs to be a script, because the downloads contain multiple manifests (separated by ---) which cannot easily concatenated into a new Kind: List manifest. So we download the files, split'em up into separate files per manifest, generate the yq load statements (because yq doesn't support wildcards in its load() function) and finally generate the List manifest with each manifest as a list item.