Replace `CloudTelemetryAuthInterceptor` with a tonic connector

We use `CloudTelemetryAuthInterceptor` as an interceptor to inject the authentication headers. The interceptors are synchronous, our credentials are async. We periodically refresh the credentials in a background thread and use a `tokio::sync::watch` to bridge the sync/async gap.

We could use a custom connector to create the `Channel`:

https://docs.rs/tonic/latest/tonic/transport/struct.Endpoint.html#method.connect_with_connector

https://docs.rs/tonic/latest/tonic/transport/struct.Channel.html#method.new

In that custom connector we can add an async layer that injects the auth headers.