Enable specifying a secret for flux git creds

chart/templates/_helpers.tpl

chart/templates/argocd/gitrepository.yaml

chart/templates/authservice/gitrepository.yaml

chart/templates/clusterauditor/gitrepository.yaml

chart/templates/gatekeeper/gitrepository.yaml

chart/templates/git-credentials.yaml

chart/templates/istio/controlplane/gitrepository.yaml

chart/templates/istio/operator/gitrepository.yaml

chart/templates/logging/eck-operator/gitrepository.yaml

...templates/logging/elasticsearch-kibana/gitrepository.yaml

chart/templates/logging/fluentbit/gitrepository.yaml

chart/templates/monitoring/gitrepository.yaml

chart/templates/twistlock/gitrepository.yaml

chart/values.yaml