Add support for model-driven CLI from Nokia SR OS

[akarneliuk]

Pre-Request Checklist

• [ ] Passes rubocop code analysis (try rubocop --auto-correct)
• [ ] Tests added or adapted (try rake test)
• [x] Changes are reflected in the documentation
• [x] User-visible changes appended to CHANGELOG.md

Description

The existing model for Nokia SR OS, which is called sros is based on the so-called classic CLI format, which Nokia is gradually phasing out and is not recommending to use for new deployments. This PR aims to add a new model called srosmd, what stands for SROS Model-Driven CLI and which unlocks possibility to backup configuration from Nokia SR OS leveraging the new model-driven CLI, which has a different syntax.

Tests are run against the real Nokia SR OS running model-driven CLI: nokia-srosmd.txt

Please, let me know if anything else is needed.

Best, Anton