Can't specify default values for LogicalType: "decimal" fields

[shawn-higgins1]

Similar issue to #198
For any field with logicalType decimals, setting a default value, throws an error.

default value ought to encode using field schema...

package main_test

import (
	"testing"

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

func TestNewCodec(t *testing.T) {
	schema := `{
    "type" : "record",
    "name" : "schema",
    "fields" : [{
        "name" : "price",
        “type”: [{
            “type”: “bytes”,
           “scale”: 2,
           “precision”: 20,
           “connect.version”: 1,
           “connect.parameters”: {
                      “scale”: “2”,
                      “connect.decimal.precision”: “20”
           },
           “connect.default”: “AA==“,
           “connect.name”: “org.apache.kafka.connect.data.Decimal”,
           “logicalType”: “decimal”
           }, “null”],
           “default”: “\u0000”
    }]
}`
	_, err := goavro.NewCodec(schema)
	if err != nil {
		t.Error(err)
	}
}

The test fails with Record "schema" field 1 ought to be valid Avro named type: Union item 2 ought to be valid Avro type: Record "*" field "price": default value ought to encode using field schema: cannot transform to bytes, expected *big.Rat, received string

Even if you try to change the schema to have a default type of *big.Rat there is no way to achieve this as the value will never get interpreted as a *big.Int. However, the Avro schema shouldn't have to work with a specific GO type it should be able to handle a default value specified in the same way as a normal avro decimal field.