gateway-api support: Implement request percentage mirroring

[sunjayBhatia]

See: percentage and fraction fields of Mirror filter: https://gateway-api.sigs.k8s.io/reference/spec/#httprequestmirrorfilter

[github-actions]

Hey @sunjayBhatia! Thanks for opening your first issue. We appreciate your contribution and welcome you to our community! We are glad to have you here and to have your input on Contour. You can also join us on our mailing list and in our channel in the Kubernetes Slack Workspace

[github-actions]

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