spanner/spannertest: support adding columns to tables with data
spanner/spannertest: support adding columns to tables with data

This is restricted to new columns without a NOT NULL qualification.

Updates #1181.

Change-Id: I30533a06fbec9de05a1383a60a7b26f198f1cc4c
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/43235
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Knut Olav Løite <62634d14bf6dff6edb4b7af974862bb227095f71@gmail.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go