Add test to require all config defined in application.yml.default

[aduth]

🛠 Summary of changes

Adds a test that requires all configuration to have a default value specified in the top-level of application.yml.default.

Why?

• So it's clearer by looking at application.yml.default all of the available configuration
  • Adds missing values:
    • lexisnexis_threatmetrix_api_key
    • lexisnexis_threatmetrix_base_url
    • lexisnexis_threatmetrix_org_id
    • lexisnexis_threatmetrix_policy
• It complements the existing "redundant configuration" by ensuring a value exists in the default scope, particularly where previously two environments may have shared the same configuration, now collapsed to a single default value in the top-level scope

Previous discussion: #11340 (comment)

📜 Testing Plan

rspec spec/lib/identity_config_spec.rb

Review that there's no effective changes in configuration in config/application.yml.default.