configdiff: T4260: add support for diff_tree class

[jestabro]

Change Summary

Add support for the configtree diff algorithm. A new function

ConfigDiff().is_node_changed(path) -> bool

is added to recursively detect changes in the tree below the node at
path; existing functions take the keyword argument 'recursive: bool' to
apply the algorithm in place of the existing, non-recursive, comparison.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4260

Component(s) name

Proposed changes

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly