fix(lib-dynamodb): support command reuse

[kuhe]

Issue

#7217

Description

This fix supports reuse of Command class instances, particularly in the special case of the @aws-sdk/lib-dynamodb DynamoDBDocumentClient.

Currently, the reuse of Command class instances is undefined behavior.

Example of command reuse:

import { DynamoDBDocumentClient, GetCommand } from "@aws-sdk/lib-dynamodb";
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";

const ddb = new DynamoDBClient();
const doc = DynamoDBDocumentClient.from(ddb);

const get = new GetCommand({ ... });

await doc.send(get);
await doc.send(get);

https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/javascript_dynamodb_code_examples.html

We had previously expected each send call to receive a new instance of the Command in question, but now recognize the possibility of reusing command instances.

Testing

added integ tests