spanner: support scanning into slice of structs []Entry

[egonelbre]

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:

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 = <the container is not a slice of struct pointers: spanner: code = \"InvalidArgument\", desc = \"type *[]main.Entry cannot be used for decoding ARRAY[STRUCT]\">"

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.