Allow setting custom headers for the fullstack dev server

[jkbz64]

What is the problem this feature would solve?

One of the common issues of building small iframe apps (where bun fullstack dev server works really well) is working around the CORS during the dev, it can be (kinda) workaround with bunx serve with correct serve.json (or spinning up nginx) + bun build --watch (without doing hacks like allowing insecure origin in the browser) but it would be nice to be able to do it directly through bun.

What is the feature you are proposing to solve the problem?

Some ideas:

• Allow adding the headers through the env variables
• Add some executable switch like --custom-header="Access-Control-Allow-Origin: *"
• bunfig.toml section
• Pull headers from meta tags e.g <meta http-equiv="Access-Control-Allow-Origin" content="*" />

What alternatives have you considered?

No response