VulnerableApp-facade: Nginx proxy rule for Mailpit UI

[preetkaran20]

Touches the facade's https://github.com/SasanLabs/VulnerableApp-facade/blob/main/nginx.conf

Add /mailpit/ location block proxying to http://mailpit:8025/
Include WebSocket upgrade headers (Upgrade, Connection) for Mailpit's live email feed
Verify Mailpit UI loads correctly through http://localhost/mailpit/

[preetkaran20]

Depends on #649

[preetkaran20]

#648 parent issue

https://github.com/SasanLabs/VulnerableApp/wiki/Design-Document:-SMTP-Server-Integration-for-VulnerableApp Design doc