spanner: support scanning into slice of structs []Entry

## Is your feature request related to a problem? Please describe.

Currently it's possibly to scan into a slice of pointers to structs, however not to a slice of structs:

``` Go
client, err := spanner.NewClient(ctx, "projects/"+ProjectID+"/instances/"+InstanceID+"/databases/"+DatabaseName)
if err != nil {
panic(err)
}
defer client.Close()

type Entry struct {
Name string
Value int64
}

input := []Entry{
{Name: "Hello", Value: 1000},
{Name: "World", Value: 2000},
}

rows := client.ReadOnlyTransaction().Query(ctx, spanner.Statement{
SQL: `SELECT ARRAY(SELECT AS STRUCT Name, Value FROM UNNEST(@input)) AS entries`,
Params: map[string]any{
"input": input,
},
})
err = rows.Do(func(row *spanner.Row) error {
var output []Entry
if err := row.Columns(&output); err != nil {
return err
}
for _, entry := range output {
fmt.Println(entry)
}
return nil
})
fmt.Fprintln(os.Stderr, err)
```

Fails with:

```
spanner: code = "InvalidArgument", desc = "failed to decode column 0, error = "
```

## Describe the solution you'd like

I would like that using the following would work:

```
var output []Entry
if err := row.Columns(&output); err != nil {
fmt.Fprintln(os.Stderr, err)
}
```

## Describe alternatives you've considered

It is possible to use:

```
var output []*Entry
if err := row.Columns(&output); err != nil {
fmt.Fprintln(os.Stderr, err)
}
```

And then later copy all the scanned values to a `[]Entry` afterwards, however it does add more unnecessary code and it might allocate more.