Add support for encoding and decoding attribute value union

* Fixes aws#895
* Fixes aws#929
* Fixes aws#115 to encode `null` for nil map/slice unless `omitempty` is
  set. Simplifies the rules of `null` vs skipped vs zero value for all
  types.

feature/dynamodb/attributevalue/convert.go

@@ -0,0 +1,89 @@
+package attributevalue
+
+import (
+	"fmt"
+
+	ddb "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
+	streams "github.com/aws/aws-sdk-go-v2/service/dynamodbstreams/types"
+)
+
+// FromDynamoDBStreamMap converts a map of Amazon DynamoDB Streams
+// AttributeValues, and all nested members, to map of Amazon DynamoDB
+// AttributeValues.
+func FromDynamoDBStreamMap(from map[string]streams.AttributeValue) (to map[string]ddb.AttributeValue, err error) {
+	to = make(map[string]ddb.AttributeValue, len(from))
+	for field, value := range from {
+		to[field], err = FromDynamoDBStream(value)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return to, nil
+}
+
+// FromDynamoDBStreamList converts a slice of Amazon DynamoDB Streams
+// AttributeValues, and all nested members, to slice of Amazon DynamoDB
+// AttributeValues.
+func FromDynamoDBStreamList(from []streams.AttributeValue) (to []ddb.AttributeValue, err error) {
+	to = make([]ddb.AttributeValue, len(from))
+	for i := 0; i < len(from); i++ {
+		to[i], err = FromDynamoDBStream(from[i])
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return to, nil
+}
+
+// FromDynamoDBStream converts an Amazon DynamoDB Streams AttributeValue, and
+// all nested members, to an Amazon DynamoDB AttributeValue.
+func FromDynamoDBStream(from streams.AttributeValue) (ddb.AttributeValue, error) {
+	switch tv := from.(type) {
+	case *streams.AttributeValueMemberNULL:
+		return &ddb.AttributeValueMemberNULL{Value: tv.Value}, nil
+
+	case *streams.AttributeValueMemberBOOL:
+		return &ddb.AttributeValueMemberBOOL{Value: tv.Value}, nil
+
+	case *streams.AttributeValueMemberB:
+		return &ddb.AttributeValueMemberB{Value: tv.Value}, nil
+
+	case *streams.AttributeValueMemberBS:
+		bs := make([][]byte, len(tv.Value))
+		for i := 0; i < len(tv.Value); i++ {
+			bs[i] = append([]byte{}, tv.Value[i]...)
+		}
+		return &ddb.AttributeValueMemberBS{Value: bs}, nil
+
+	case *streams.AttributeValueMemberN:
+		return &ddb.AttributeValueMemberN{Value: tv.Value}, nil
+
+	case *streams.AttributeValueMemberNS:
+		return &ddb.AttributeValueMemberNS{Value: append([]string{}, tv.Value...)}, nil
+
+	case *streams.AttributeValueMemberS:
+		return &ddb.AttributeValueMemberS{Value: tv.Value}, nil
+
+	case *streams.AttributeValueMemberSS:
+		return &ddb.AttributeValueMemberSS{Value: append([]string{}, tv.Value...)}, nil
+
+	case *streams.AttributeValueMemberL:
+		values, err := FromDynamoDBStreamList(tv.Value)
+		if err != nil {
+			return nil, err
+		}
+		return &ddb.AttributeValueMemberL{Value: values}, nil
+
+	case *streams.AttributeValueMemberM:
+		values, err := FromDynamoDBStreamMap(tv.Value)
+		if err != nil {
+			return nil, err
+		}
+		return &ddb.AttributeValueMemberM{Value: values}, nil
+
+	default:
+		return nil, fmt.Errorf("unknown Amazon DynamoDB Streams AttributeValue union member, %T", from)
+	}
+}