fix(IAM): endpoint in aws-iso partition

[pinak]

Fix endpoint for IAM for us-iso partition

IAM is partitional service, previously it was being treated as regional.

Checklist

• npm run test passes -> tests don't even pass in master branch for me locally
• changelog is added, npm run add-change
• run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed

[trivikr]

Verified that correct endpoint is logged for IAM

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

const region = "us-iso-west-1";
const client = new AWS.IAM({ region });

// Logs iam.us-iso-east-1.c2s.ic.gov
console.log(`JS v2 endpoint for region '${region}':`, client.endpoint.host);