stream_curl: add libcurl-based stream backend

Introduces stream_http, an internal libcurl-driven backend for http://
and https:// URLs. Runs all transfers on a dedicated curl multi thread
with a producer-side ring buffer, and exposes HTTP/2 multiplexing,
QUIC/HTTP/3 (when libcurl supports it), HSTS and TCP keep-alive.

Should generally be more stable and faster than FFmpeg HTTP/1.1 impl,
additionally connections are kept-alive between files open, so if you
open playlist of network files and navigate through them, it will re-use
the same connection.

Build is gated on the new 'libcurl' meson option (auto). When disabled
or unavailable, mpv silently falls back to FFmpeg's HTTP implementation.