Adds a RedisCache config block and redisCache implementation for the
generic cache.Cache interface, enabling distributed HTTP response
caching across gost instances.
go-redis/redis/v8 was already a dependency (used by recorder and
internal loader) — no new imports needed.
Implements the cache.Cache interface with in-memory backend (TTL,
maxSize/maxBytes bounds, oldest/LRU eviction, background cleanup).
Adds httpcache layer for HTTP response policy (per-status TTL,
serve-stale, method/body filtering). Wires into config parser,
loader, registry, and all four sniffing handlers (forward/local,
forward/remote, sni, http) so HTTP responses are cached before
dialing upstream.
Bumps core dependency to v0.6.0 for the new cache.Cache interface.