feat: add SetResolver and SetHosts for custom DNS (#169)

[ManuelReschke]

Add SetResolver and SetHosts for custom DNS on HTTP/1 and HTTP/2

Fixes #169.
Closes #168 (discussion).

Summary

Adds a simple way to customize DNS resolution for HTTP/1 and HTTP/2, addressing long DNS timeouts when crawling with a known host list.

• Client.SetResolver(*net.Resolver) — use a custom net.Resolver (e.g. a specific DNS server) via SetDial + net.Dialer.
• Client.SetHosts(map[string]string) — static hostname→IP mapping (hosts-file style). Unlisted hostnames fail immediately with "no such host" and do not hit the system resolver.
• Global wrappers: SetResolver, SetHosts.
• HTTP/2 fix: when DialContext is set, the HTTP/2 transport uses it for the underlying TCP dial (including EnableForceHTTP2), with TLS handshake timeout/trace handling aligned to HTTP/1 addTLS.

Motivation

From discussion #168: crawlers that hit many domains pay for system DNS timeouts on dead hosts. A custom resolver or hosts map that can return NOHOST quickly covers the common case.

SetDial already allowed this, but a dedicated API is clearer and documents the intended pattern.

API

// Custom DNS server
r := &net.Resolver{
	PreferGo: true,
	Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
		var d net.Dialer
		return d.DialContext(ctx, "udp", "1.1.1.1:53")
	},
}
client := req.C().SetResolver(r)

// Static hosts map (fail closed for unknown hostnames)
client := req.C().SetHosts(map[string]string{
	"api.internal": "10.0.0.5",
	"db.internal":  "10.0.0.6",
	"v6.internal":  "::1", // brackets like "[::1]" also accepted
})

Notes:

• Only applies to HTTP/1 and HTTP/2 (same scope as SetDial). HTTP/3 is unchanged.
• Host matching is case-insensitive and IDNA-normalized.
• Map values must be literal IPs; non-IPs never fall through to system DNS (clear error instead). IPv6 brackets are normalized.
• IP-literal request URLs (e.g. https://1.2.3.4/) skip the map and dial directly.
• Empty/nil map → every non-literal hostname fails closed.
• SetDialTLS still bypasses this dialer for HTTPS when set.
• SetDial, SetResolver, SetHosts, and SetUnixSocket replace each other (last call wins).
• The map is copied.

Tests

• SetResolver: IP URL works with a broken custom resolver; hostname fails with that resolver’s error.
• SetHosts: successful HTTP/1 and forced HTTP/2 via mapped hostname.
• Unknown host / empty map fail fast with "no such host".
• IP-literal passthrough, invalid IP (no DNS), IPv6 bracket normalization, IDNA key normalization.
• Caller map mutation ignored; SetDial replaces hosts dialer.

Validation:

go test ./...
go vet ./...
git diff --check