mobile: stats cluster has empty SNI

Tracking an issue found during enabling plaform-based cert validator for stats cluster by Lyft.

If `addGrpcStatsDomain()` is used, the connection to the gRPC stats server has empty SNI during the TLS handshake.

Fix: configuring `auto_sni` in typed_extension_protocol_options config won't work as the Host header of an HTTP request doesn't have a valid DNS domain name. So a potential fix would need to set `sni` with the specified DNS domain in `UpstreamTlsContext` config.