Enable the Web IDE Extension Marketplace for Self-managed
## Description
This epic is focused on the MVC required to enable [`web_ide_extensions_marketplace`](https://gitlab.com/gitlab-org/gitlab/-/issues/459028) for self-managed by default.
## Requirements
- **Use case:** Self-managed instance admin can enable/disable Extensions Marketplace through admin settings UI
- **Use case:** Self-managed instance admin can choose to use OpenVSX default settings through admin settings UI
- **Use case:** Self-managed instance admin can choose custom Extensions Marketplace values through admin settings UI
- These configuration use cases are all free-tier features
- We are **not** looking at Group-level customization in this epic
**NOTE:** Before closing this off, check the https://gitlab.com/groups/gitlab-org/-/epics/15398+ for any issues that might need to be included here.
## UX Questions
- Should the Extensions Marketplace URL's input be 1 JSON field or a bunch of fields?
- `product_url`
- `resource_url`
- `X_url`
epic