service/dynamodb/expression: Allow AttributeValue as a value to BuildOperand.

[bullgare]

Now you have no way to specify a dynamodb.AttributeValue as a parameter to expression.BuildOperand method.
If you do so, it will be treated as any other struct, and instead of just passing a value, it will Marshal it's inner struct (all the {...BOOL: true, NULL: true, ...} fields that are needed for inner implementation).

That is a problem when you (for example) have a struct that was already been Marshalled:

package main

import (
	"fmt"

	"github.com/aws/aws-sdk-go/service/dynamodb"
	"github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
	"github.com/aws/aws-sdk-go/service/dynamodb/expression"
)

type T struct {
	ID    uint64  `dynamodbav:"id"`
	Code  string  `dynamodbav:"code"`
	Total float32 `dynamodbav:"amount"`
}

func main() {
	t := T{
		ID:    1,
		Code:  "code",
		Total: 10.5,
		// it can have dozens of fields
	}

	// it is now Marshalled into proper structs
	av, err := dynamodbattribute.MarshalMap(t)
	if err != nil {
		panic(err)
	}

	updateExpr := buildUpdateExpressionFrom(av)

	builder := expression.NewBuilder().
		WithUpdate(updateExpr)

	expr, err := builder.Build()
	if err != nil {
		panic(err)
	}

	fmt.Printf("%q", expr) // it will have a mess now. fixed in this PR
}

func buildUpdateExpressionFrom(av map[string]*dynamodb.AttributeValue) expression.UpdateBuilder {
	var expr expression.UpdateBuilder

	for attr, value := range av {
		expr = expr.Set(expression.Name(attr), expression.Value(value)) // <--- here is a problem now
	}

	return expr
}

[bullgare]

It can not be done on consumer side as expression.OperandBuilder interface expects BuildOperand method which works with exact implementation of expression.Operand which has unexportalbe fields.