dynamodb/expression: Value doesn't work as expected with dynamodb.AttributeValue

[sudhirj]

Confirm by changing [ ] to [x] below to ensure that it's a bug:

• I've gone though the API reference
• I've checked AWS Forums and StackOverflow for answers
• I've searched for previous similar issues and didn't find any solution

Describe the bug
Calling expression.Value() with a dynamodb.AttributeValue dumps the full JSON structure instead of using the given value.

Version of AWS SDK for Go?
Latest

Version of Go (go version)?
Latest

To Reproduce (observed behavior)

update.Set(
			expression.Name("key"),
			expression.Value(dynamodb.AttributeValue{
				S: aws.String("value"),
			}),
		)

Expected behavior
I'd expect the value to be set correctly typed to a string.

Additional context
There seems to be type based coercion going on to put the value into an AttributeValue, but if it already is one we should be able to just take it as-is.

[sudhirj]

Specifically, here we can either avoid calling dynamodbattribute.Marshal on the values if they already of type dynamodb.AttributeValue, or we can do the check inside the encode method.

I'm happy to raise a PR if the maintainers can specify which approach would be more agreeable.

[github-actions]

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