nmtui: add veth page
Summary
Adds a nmtui page for configuring veth interfaces.
Purpose
Before this, nmtui offered an option to configure a veth interface, but crashed due to the fact that there is no implemented page for veth. Related issue: #1507
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
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the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable -
the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.