spanner_v1/trace: allow option for extended tracing which will allow recording the SQL statement if provided

[odeke-em]

Part of modernized tracing is that it becomes pretty useful for customers to investigate slow queries when observing traces; customers would like to opt-in to ensure that this library records their SQL statements in their traces where applicable e.g. if they've explicitly opted into having their spans annotated with the SQL statement; explicitly because some SQL statement might have personally identifiable information (PII) and leak sensitive data out.

I am going to work on this issue myself.