chore(kuma-cp): add weights to xds.Cluster interface

[bartsmykla]

This setting is necessary to allow MeshTCPRoute to create "splits" without the need of using deprecated abstractions like ClusterImpl. I also think it's quite elegant solution which allows to remove type assertion to ClusterImpl in TcpProxyConfigurator, when there are more clusters provided as parameters.

Checklist prior to review

• Link to relevant issue as well as docs and UI issues
  • MeshTCPRoute implementation #6787
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS
  • it won't
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • tested manually
• Do you need to update UPGRADE.md?
  • there is no need, as so use this feature, it has to be explicit
• Does it need to be backported according to the backporting policy?
  • it doesn't
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?
  • there is no need