Ping public DNS from the router UI to check connectivity

[pascalzauberzeug]

Motivation

The Teltonika router UI offered no way to verify whether the device actually has working internet, only whether interfaces are up.
This made diagnosing connectivity issues in the field cumbersome — interfaces can report healthy while DNS or upstream routing is broken.

Implementation

Adds a "Check Internet"-button to the Teltonika ui and the underlying functionality to the Teltonika hardware class.

Also converted the class-level tuples to frozenset since they are only used for membership checks.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• I chose meaningful labels (if GitHub allows me to so).
• I documented breaking changes and set the 'breaking change' label if needed
• The implementation is complete.
• Tests with a real hardware have been successful (or are not necessary).
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).