Write a test to ensure that composer plugins match in composer templates

Problem/Motivation

See #3294205: [Packaging broken!] Composer v2.2 prompts to authorize another plugin when stability=dev → and 📌 Add open-telemetry/sdk and open-telemetry/exporter-otlp as dev dependencies Active where we broke this.

Write a test to ensure that the three match.

Steps to reproduce

Proposed resolution

The allow-plugins section in these three files need to match:
composer.json
composer/Template/LegacyProject/composer.json
composer/Template/RecommendedProject/composer.json

Discussing with @longwave, we thought about manually parsing the three composer.json files then checking those bits, this is a bit hardcoded but we don't want to recreate the packaging pipeline in core and there may be precedent already.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet