[feature] SC-166737/improve app proxy security by restricting where token replacements can go

[HappyPaul55]

This pull request introduces an update to the Mailchimp API configuration in the manifest.json file. The main change is the addition of a settingsInjection property to improve how sensitive settings like OAuth credentials are handled.

Configuration improvements for Mailchimp API:

• Added an empty settingsInjection object to the main Mailchimp API endpoint configuration to support future extensibility.
• Added a settingsInjection object to the OAuth endpoint configuration, specifying that client_id and client_secret should be injected into the request body for authentication purposes.

[Copilot]

Pull Request Overview

This PR enhances security for the Mailchimp app by adding settingsInjection configuration to the proxy whitelist, ensuring sensitive OAuth credentials are properly injected into API requests in a controlled manner.

Key Changes:

• Added settingsInjection configuration to control how sensitive settings are injected into proxy requests
• Configured OAuth endpoint to inject client_id and client_secret into request body fields
• Added empty settingsInjection object to main API endpoint for future extensibility

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[github-actions]

Build for commit 8bebaf2 deployed to: https://mailchimp-pr-63.ci.next.deskprodemo.com

URLs:
https://mailchimp-pr-63.ci.next.deskprodemo.com/horizon-ui/app