[lambda][flare] Update site endpoints

[nhulston]

What and why?

Due to a DNS inconsistency, the lambda flare endpoint doesn't work on sites with no subdomain (for example, US1).
This PR updates sites to append endpoint URL with app. when no subdomain is present in the original URL.

How?

if ([DATADOG_SITE_US1, DATADOG_SITE_EU1, DATADOG_SITE_GOV].includes(baseUrl)) {
  endpointUrl = 'app.' + baseUrl
}

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
  Snapshots have been updated to handle this change

[datadog-datadog-prod-us1]

Datadog Report

Branch report: nicholas.hulston/serverless-flare/update-sites
Commit report: 92f64cc

✅ datadog-ci-tests: 0 Failed, 0 New Flaky, 873 Passed, 0 Skipped, 59.29s Wall Time