k8s TLS secret credential provider

On k8s (and other environments) secrets have an independent pipeline, e.g. k8s Secrets API. Secrets must be served independently from the rest of Envoy configuration because of the higher security expectations for credentials in-transit and at-rest over the regular xDS. Currently, this requires running an agent next to Envoy to convert from the platform secrets API to SDS, which adds a runtime overhead per instance. The feature request is to integrate with those platform APIs [directly](https://kubernetes.io/docs/tasks/run-application/access-api-from-pod/#without-using-a-proxy) via the new credential provider functionality, bypassing SDS.