vyconf: T5083: add childRequirement to reference tree

[thomasfinstad]

Change Summary

Implements validation of node children, allowing for declarative validation through XML schemas

NOTE: remember to change the container library pinning: https://github.com/vyos/vyos-build/blob/257496d6bf022c36bd13651b247921b2f22c6db6/docker/Dockerfile#L156

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)
• Other (please describe): Added test case

Related Task(s)

https://vyos.dev/T5083

Related PR(s)

vyos/vyos-1x#3575

Proposed changes

How to test

OPAM must be setup to function properly, and dependencies must be installed.

apt-get install -y opam
opam init --auto-setup --disable-sandboxing
eval $(opam env)
make deps

Makefile runs the test case

# make test
+ dune clean
+ rm -rf test/testresults
+ opam install -y .
[WARNING] Running as root is not recommended
[NOTE] Ignoring uncommitted changes in /workspaces/vyos1x-config (`--working-dir' not active).
[vyos1x-config.0.2] synchronised (git+file:///workspaces/vyos1x-config#T5083)
The following actions will be performed:
  - recompile vyos1x-config 0.2*

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> removed   vyos1x-config.0.2
-> installed vyos1x-config.0.2
Done.
+ dune build ./test/
+ cd _build/default/test/             
+ dune exec ./test_vyos1x.bc
Entering directory '/workspaces/vyos1x-config'
Leaving directory '/workspaces/vyos1x-config'
Testing `Test Suite'.                
This run has ID `FS2CCJWK'.

  [OK]          InterfaceDefinitions          0   can greet Tom.
  [OK]          InterfaceDefinitions          1   Generate xml to json cache.

Full test results in `/workspaces/vyos1x-config/_build/default/test/_build/_tests/Test Suite'.
Test Successful in 0.005s. 2 tests run.
+ cd /workspaces/vyos1x-config

Checklist:

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

[jestabro]

See comments in companion PR:
vyos/vyos-1x#3575 (review)
This is nice work, inviting further discussion.