feat: allow custom changelog sections

[tanguyantoine]

This pull request adds support for customizing changelog sections at the workflow level in the release-please-action, allowing users to override changelog formatting for all packages directly from the workflow file. It introduces a new changelog-sections input, updates documentation and action metadata, and ensures that these overrides are applied consistently. The changes are covered by new and updated tests.

Changelog customization support:

• Added a new optional changelog-sections input to the action (action.yml, README.md). This input allows users to specify a JSON array of changelog section configurations, overriding settings from the config file for all packages. [1] [2] [3]
• Updated the documentation to explain how to use the new changelog-sections input, including examples and descriptions in the input table. [1] [2]

Implementation in action logic:

• Parsed the changelog-sections input and passed it through the action's logic, ensuring it is applied to all repository configs in the manifest, both when building from config and when loading an existing manifest. [1] [2] [3] [4] [5] [6]

Testing improvements:

• Added and updated tests to verify that changelog-sections can be specified, is parsed correctly, and is applied to all repository configs, both with and without a release type. [1] [2]
• Improved code style and consistency in test files and main action logic (e.g., arrow function formatting, trailing commas, and assertion formatting). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]