feat: expose Payload.WritePayload to allow serializing into IPC format (#421)

### Rationale for this change
The BigQuery Storage Write API now accepts Arrow data. But schema data
and record batches needs to send separately. Right now the
`ipc.NewWriter` writes the schema data to the output buffer, which makes
it fail on the BigQuery side, as the first message is the schema. So we
need a way to write just RecordBatches.

See related discussion on
googleapis/google-cloud-go#12478

### What changes are included in this PR?
Add `WritePayload ` method to `Payload`, which exposes the underlying
`writeIPCPayload` method, allowing to write IPC messages more easily.

### Are these changes tested?
Yes, tested with BigQuery Storage Write API. But also added local tests
to it too.

### Are there any user-facing changes?
Yes, `WritePayload` will be a public method available option to users.

Author: alvarowolfx

arrow/ipc/file_writer.go

@@ -203,6 +203,12 @@ func (p *Payload) SerializeBody(w io.Writer) error {
 	return nil
 }
 
+// WritePayload serializes the payload in IPC format
+// into the provided writer.
+func (p *Payload) WritePayload(w io.Writer) (int, error) {
+	return writeIPCPayload(w, *p)
+}
+
 func (p *Payload) Release() {
 	if p.meta != nil {
 		p.meta.Release()

arrow/ipc/writer_test.go

@@ -332,3 +332,28 @@ func TestGetPayloads(t *testing.T) {
 
 	assert.Truef(t, array.RecordEqual(rec, got), "expected: %s\ngot: %s", rec, got)
 }
+
+func TestWritePayload(t *testing.T) {
+	mem := memory.NewCheckedAllocator(memory.DefaultAllocator)
+	defer mem.AssertSize(t, 0)
+
+	bldr := array.NewRecordBuilder(mem, arrow.NewSchema([]arrow.Field{{Name: "col", Type: arrow.PrimitiveTypes.Int8}}, nil))
+	bldr.Field(0).(*array.Int8Builder).AppendValues([]int8{1, 2, 3, 4, 5}, nil)
+	rec := bldr.NewRecord()
+	defer rec.Release()
+
+	var buf bytes.Buffer
+	p, err := GetRecordBatchPayload(rec, WithAllocator(mem))
+	defer p.Release()
+	require.NoError(t, err)
+
+	_, err = p.WritePayload(&buf)
+	require.NoError(t, err)
+
+	r := NewMessageReader(&buf, WithAllocator(mem))
+	defer r.Release()
+
+	msg, err := r.Message()
+	require.NoError(t, err)
+	require.True(t, msg.Type() == MessageRecordBatch)
+}