feat: add batchGetItem types

[brendo-m]

Basic implementation, seems to work. It doesn't quite fit with the spirit of the library though because you can call batchGetItem for multiple tables. Should we impose that TypesafeDynamoDBv{2,3} only operates on a single table? if so, parameterizing the table name would improve the typing of the response here.

import { TypeSafeDynamoDBv2 } from "typesafe-dynamodb/lib/client-v2";

const typesafeClient: TypeSafeDynamoDBv2<"MyTableName", Record, "key", "sort"> = client;
const response = await table.batchGetItem({...}).promise();
const responseTable = response.Responses!.MyTableName; // type checks

[sam-goodwin]

The problem with TableName is that developers don't usually hard code literal values. Can we still narrow in that case without the literal string value? They would have to cast:

type MyTableName<Prefix extends string = string> = `MyTable-${Prefix}`

const myTableName = process.env.MyTableName as MyTableName

[sam-goodwin]

Perhaps we should provide a type safe BatchGetItemEntry (whatever its name is) for the specific table. Then use that function like how we overrode commands.

const GetMyTableItem = TypeSafeGetItem<..>;)

client.batchGet({
  Items: [GetMyTableItem(..)]
})

[brendo-m]

The problem with TableName is that developers don't usually hard code literal values. Can we still narrow in that case without the literal string value? They would have to cast:

Good point

[brendo-m]

Pushed a fix for the misplaced ProjectionExpression, and the response needed to have ToAttributeMap<...>

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