Add httpHeaders to @gitlab/web-ide `start`

What does this MR do and why?

This MR adds the necessary httpHeaders option to our start for @gitlab/web-ide.

Screenshots or screen recordings

Before	After
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How to set up and validate locally

1. Enable :vscode_web_ide feature flag
2. Visit a project and open the Web IDE
3. Make a change and then visit the Source Control tab in the Web IDE
4. Commit and follow the prompts to commit to a new branch

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.