Decimal logical type not serilized correctly

[olegKoshmeliuk]

In current implementation decimal is serilized as simple string.
but in 1.10.2 AVRO spec it must be sterilized as:

The byte array must contain the two's-complement representation of the unscaled integer value in big-endian byte order.
The scale is fixed, and is specified using an attribute.

Example:

package main

import (
	"fmt"

	"github.com/linkedin/goavro/v2"
)

func main() {
	codec, err := goavro.NewCodec(`
        {"type":"bytes","logicalType":"decimal","precision":4,"scale":2}`)
	if err != nil {
		fmt.Println(err)
	}
	textual := []byte(`"40.20"`)
	native, _, err := codec.NativeFromTextual(textual)
	if err != nil {
		fmt.Println(err)
	}
	// Here is incorrect result. decimal value must be represented as bytes and not as string.
	binary, err := codec.BinaryFromNative(nil, native)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(string(binary))
}