refactor GenerateResourceRequirements and provide unit tests (#1822)

charts/postgres-operator/crds/postgresqls.yaml

docs/reference/cluster_manifest.md

docs/reference/operator_parameters.md

manifests/postgresql.crd.yaml

pkg/apis/acid.zalan.do/v1/crds.go

pkg/apis/acid.zalan.do/v1/postgresql_type.go

pkg/apis/acid.zalan.do/v1/zz_generated.deepcopy.go

pkg/cluster/cluster.go

pkg/cluster/connection_pooler.go

pkg/cluster/k8sres.go

pkg/cluster/k8sres_test.go

pkg/cluster/sync.go

pkg/cluster/util.go