Partial fix for bugs

[n0vad3v]

This PR aims to fix the following bugs:

• Remote server sendfile missing / when using environment var WEBP_IMAGE_PATH #383 caused by path.Join rather than url.JoinPath, making https://xxxx/path/to/join becoming https:/xxx/path/to/join
• Browsers with limited format support cannot display images in proxy mode due to missing format fallback #392 caused by incomplete and junky metadata design.
• Tries to preserve remote file extension that will correctly render Content-type when using "ALLOWED_TYPES": ["*"] and fetching non-image from remote, related issue: Incorrect Content-Type returned when requesting non-images from remote backend #388

[github-actions]

Report Summary

┌───────────────────────────────────────────────┬──────────┬─────────────────┬─────────┐
│                    Target                     │   Type   │ Vulnerabilities │ Secrets │
├───────────────────────────────────────────────┼──────────┼─────────────────┼─────────┤
│ ghcr.io/webp-sh/webp_server_go (debian 12.10) │  debian  │        0        │    -    │
├───────────────────────────────────────────────┼──────────┼─────────────────┼─────────┤
│ usr/bin/webp-server                           │ gobinary │        0        │    -    │
└───────────────────────────────────────────────┴──────────┴─────────────────┴─────────┘
Legend:
- '-': Not scanned
- '0': Clean (no security findings detected)