Support for specifying which Undefined Type to use

[ohardeng]

Currently only the default Undefined and StrictUndefined are supported, but I needed ChainableUndefined in my usecase for parsing deployment templates normally run with ansible.

This PR adds the --undefined option to allow you to select which behaviour you want for your undefineds.

Minimal example:

test_template.yml

---
apiVersion: test
spec:
  env:
    - name: ENVIRONMENT_OPENTELEMETRY_SDK_DISABLED
      value: "{{ opentelemetry.sdk.disabled | default(true) }}"
    - name: ENVIRONMENT_OPENTELEMETRY_EXPORTER_OTLP_ENDPOINT
      value: "{{ opentelemetry.exporter.otlp.endpoint | default('') }}"
    - name: APP_NAME
      value: "{{ app_name }}"

test_vars.yml

app_name: test-app

Currently this fails

jinja2 test_template.yml test_vars.yml
UndefinedError: 'opentelemetry' is undefined

With this PR it succeds

python3 -m jinja2cli test_template.yml test_vars.yml -f yaml --undefined=ChainableUndefined
---
apiVersion: test
spec:
  env:
    - name: ENVIRONMENT_OPENTELEMETRY_SDK_DISABLED
      value: "True"
    - name: ENVIRONMENT_OPENTELEMETRY_EXPORTER_OTLP_ENDPOINT
      value: ""
    - name: APP_NAME
      value: "test-app"