Set sign_with_root via Caddyfile

[tweimann]

Please make the sign_with_root option from the JSON config available in the Caddyfile
Maybe related: #6290 (comment)

[Ranveer777]

@mholt @francislavoie I would like to work on this issue.

Well, a little late to say, as I have already created a pull request.

Also, I'm currently in the process of transitioning into open source contribution with caddyserver being my first contribution. I am eager to learn and contribute to meaningful projects in this space.

[mholt]

Welcome to the project @Ranveer777 !

[Ranveer777]

@mholt I'm excited to be part of the project and look forward to contributing.