[Feature] Add HTTP content negotiation framework

[Feature] Add HTTP content negotiation framework

Add content type negotiation based on Accept header for HTTP responses.
This allows clients like DataDog's OpenMetrics scraper to receive
responses with Content-Type matching their Accept header preferences.

- Add http_content_negotiation.c/h with Accept header parsing
- Support quality factors (q=) in Accept header
- Parse Accept-Encoding for gzip/zstd/deflate support
- Add rspamd_controller_send_openmetrics_negotiated()
- Update /metrics endpoint to negotiate Content-Type
- Fallback to text/plain for Prometheus 0.0.4 compatibility