[java-spanner] Errorprone adoption

Adopt the [errorprone plugin](https://errorprone.info/) as described at https://errorprone.info/docs/installation#maven. The warnings can be addressed using the `@SuppressWarnings` annotation, or have entire classes of warnings turned off.

This will improve code quality and reduce drift with Google's monorepo. This will also enable other interesting features, such as using `@CompileTimeConstant` on some of the parameters of the statement sql builder, to encouraging parameterised queries to lower the risk of introducing SQL injection vulnerabilities.