fix error about invalid certificate when calling append_rows api

[imor]

When running the append_rows api, the following error was thrown:

Error: TonicTransportError(tonic::transport::Error(Transport, ConnectError(Custom { kind: InvalidData, error: InvalidCertificate(UnknownIssuer) })))

This was because tonic made a breaking change in version 0.12.0. To fix this we'd need to call with_native_roots method on the ClientTlsConfig object.

[lquerel]

Thanks @imor for this fix. I have time this week-end and should be able to publish a new version soon.