feat: add stage.useragent

[pi12138]

PR Description

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

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[clayton-cornell]

This will need documentation. The loki.process stage docs are here: https://grafana.com/docs/alloy/latest/reference/components/loki/loki.process/ and in the repo at /docs/sources/reference/components/loki/loki.process.md