Replace nginx with Caddy

[markbotterill]

This PR aims to replace nginx with Caddy. Caddy requires fewer files to operate, is easier to get started with and performs similarly to nginx for most use cases.

The primary query I had about this PR was to do with global variables: In the Caddyfile, Caddy expects to see a domain name (without https:// proceeding it) but the web_server requires the full URL for things like Oauth re-directs. My current solution was the addition of the $DOMAIN variable (which is then used in the $WEB_DOMAIN variable) and this can be found in the env.prod.template but may not be best practice. Very open to feedback :)

Cheers!

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[yuhongsun96]

Hello, sorry for delayed response on this one. Teams are generally more familiar with nginx and it's more performant. To support more teams being able to manage their own nginx and higher scale deployments of Danswer we will be continuing with nginx.