Create mfe_config to frontend-base conversion endpoint

[arbrandes]

Description

Create a new sister endpoint to mfe_config that reads any existing configuration data (currently django settings or site configuration) and outputs it in the format expected by frontend-base. See openedx/frontend-base#118.

TODO

• feat: add FrontendSiteConfigView for frontend-base site configuration #38061
• feat: support commonAppConfig site config option frontend-base#179
• Add runtimeConfigJsonUrl to site.config.dev.tsx in Authn
• Add runtimeConfigJsonUrl to site.config.dev.tsx in Learner Dashboard
• Add runtimeConfigJsonUrl to site.config.[dev|build].tsx in frontend-template-site
• Add runtimeConfigJsonUrl to site.config.dev.tsx in frontend-template-app