spansql: support struct

Currently spansql does not support the struct literal as noted from [this comment](https://github.com/googleapis/google-cloud-go/blob/main/spanner/spansql/parser.go#L818) [6 years ago](https://github.com/googleapis/google-cloud-go/commit/196193034a15f84dc3d3c27901990e8be77fca85)

It would be good to add this support