feat: add meaningful User-Agent header to outgoing HTTP requests

[thegoddo]

What's the problem this PR addresses?

Currently, Yarn 4 identifies itself with the default got user-agent string (got (https://github.com/sindresorhus/got)) instead of identifying itself as Yarn. This makes it impossible for sysadmins and DevOps engineers to distinguish Yarn traffic or enforce package manager allow-lists across an organization.

Closes #7149

How did you fix it?

Modified packages/yarnpkg-core/sources/httpUtils.ts inside the requestImpl function.

• Imported YarnVersion from the core constants.
• Formulated a meaningful User-Agent string following the standard convention: yarn/${yarnVersion} node/${process.version}.
• Injected it into gotOptions.headers while utilizing the spread operator (...headers) to ensure any explicitly passed custom headers are preserved and not overwritten.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.