Add endpoint override for us-isof-* region

[trivikr]

Internal JS-5148

Testing

import AWS from "../aws-sdk-js/lib/aws.js";

const client = new AWS.DynamoDB({ region: "us-isof-east-1" });
console.log(client.endpoint.host);

The output is dynamodb.us-isof-east-1.csp.hci.ic.gov

Checklist

• changelog is added, npm run add-change