feat(api): add meshexternalservice TargetRefKind and changed int to Port

[lukidzi]

Checklist prior to review

• changed port type from int to Port
• added new TargetRefKind: MeshExternalService

• Link to relevant issue as well as docs and UI issues --
• 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 --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
  • Don't forget ci/ labels to run additional/fewer tests
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label) --

Changelog: feat(MeshExternalService): implement a new resource

[slonka]

nitpick: this also brings MeshExternalService kind