Remote Execution: Make TLS and client certificate optional for storage service

For testing and local deployments, it may make sense to use insecure connections to the storage service or use TLS without client certificate/authentication. Let's make the storage service configuration consistent with the artifact server configuration, which already supports both secure and insecure connections.

For example, it might be useful to use the remote execution API for building in a local VM or emulator for cross-platform builds. Mandating TLS may add unnecessary setup and runtime overhead.