Add validation in decimal to numeric conversion to ensure precision and recall is supported by spanner.

read more here: https://cloud.google.com/spanner/docs/data-types#numeric_type.

Similar validations are present in other clients eg: https://github.com/googleapis/java-spanner/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Value.java#L141-L155