Add timeout and connect timeout for spanner connections

[ivankelly]

We encountered an issue with a broken spanner instance where when we connected the process just sat there doing nothing. Further investigation showed it was a server side problem. Other clients failed to connect also. But when the other clients failed they gave a error that the request to create sessions had timed out.

The rust client should do this also. I've added default timeouts for the spanner client based on the values in https://github.com/googleapis/google-cloud-go/blob/main/spanner/apiv1/spanner_client.go#L65 In the go client, some requests take a higher timeout, but the structure of the rust client doesn't allow this, so I set both defaults to 30s. Other users can increase, via channel_config, if they find this to be too low.

I didn't set defaults for pubsub as we don't use it.

[yoshidan]

Could you run cargo fmt to fix CI.

[ivankelly]

Done. I was sure I had run it before pushing :/

[yoshidan]

Thanks!