Add new billing settings endpoint

What does this MR do?

The aim of this is to allow reconciliation retries when processing credit cards, and this MermaidJS diagram really helps get the point across. (tip: type e to see the yaml in MR Edit mode)

gantt
    title A Gantt Diagram for Payment Processor
    dateFormat  YYYY-MM-DD
    section Section
    A task           :a1, 2014-01-01, 30d
    Another task     :after a1  , 20d
    section Another
    Task in sec      :2014-01-12  , 12d
    another task      : 22d

Related issues

• Auto DevOps: Enabling Auto DevOps | Template | Future CI/CD Catalog Direction
• Code Owners: File in the Repo | (Scroll down in MR for inclusion) | Direction | Dogfood
• Security Policies | IaC Vulnerabilities
• GitLab is a CVE Numbering Authority and manages its own Advisories Database
• Native Form Support

Author's checklist (required)

• Relate all corresponding issues
• Write appropriate unit tests
• Write solution
• Preview changes and ensure Unit/Regression Tests are Running and Passing
• Add change to CHANGELOG.md
• Assign to Reviewer

Reviewer's checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.
• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
• Assign to Maintainer

Closes #2

Future Iterations

• Consider List of Engines