Refactor(plugins): Improve schema models

[ClausHolbechArista]

• Add a few extra methods to schema classes to improve code readability on future refactored code.
  • provide ways of updating or adding new items inline, without having to give the full class path.
  • ._update on AvdModel and append_new on AvdList and AvdIndexedList gives type hints similar to __init__() of the item class. This means it is easy to provide the correct supported fields.
  • The new methods return the instance to make it easier to chain with other methods.

[github-actions]

Review docs on Read the Docs

To test this pull request:

# Create virtual environment for this testing below the current directory
python -m venv test-avd-pr-4795
# Activate the virtual environment
source test-avd-pr-4795/bin/activate
# Install all requirements including PyAVD
pip install "pyavd[ansible] @ git+https://github.com/ClausHolbechArista/avd.git@refactor/plugins/improve-schema-models#subdirectory=python-avd" --force
# Point Ansible collections path to the Python virtual environment
export ANSIBLE_COLLECTIONS_PATH=$VIRTUAL_ENV/ansible_collections
# Install Ansible collection
ansible-galaxy collection install git+https://github.com/ClausHolbechArista/avd.git#/ansible_collections/arista/avd/,refactor/plugins/improve-schema-models --force
# Optional: Install AVD examples
cd test-avd-pr-4795
ansible-playbook arista.avd.install_examples

[emilarista]

Tested, LGTM!

I pushed some extra changes. Somehow it messed up so I will dismiss your review.

[emilarista]

Retested with same repos, still LGTM!

[gmuloc]

In general I feel like it would make sense to add some unittest for merging and inheriting with the flags for some classes to make sure we don't break this in further refactoring