feat: add new endpoint that uses flags and accountId

[lemusthelroy]

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Summary

Integrations are changing and this also means the endpoint we use to fetch information about integrations installed on a site is also changing. This points to the new endpoint behind a feature flag.

Changes will also need to be made in buildbot to pass the accountId and a PR is due to merge in the CLI to support passing the necessary feature flags netlify/cli#6691

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[Skn0tt]

I believe you'll also need to add the accountId flag here:

build/packages/config/src/bin/flags.ts

Line 122 in e4e9787

siteId: {