plugin/https: Add max_streams to limit HTTP/2 concurrent streams (#8522)

Add a max_streams option to the *https* plugin to limit the number of
concurrent HTTP/2 streams per DoH connection. This lets operators cap
per-connection concurrency (guarding against resource exhaustion) or
raise it above the Go default for high-fan-in clients that multiplex
many requests over a single connection.

Semantics match the existing *https3* plugin's max_streams:
- omitted  -> Go HTTP/2 server default is used
- 0        -> use the underlying HTTP/2 transport default
- positive -> advertise exactly that many concurrent streams
- negative -> rejected at config parse time

The limit is applied via the standard library http.Server.HTTP2
(HTTP2Config.MaxConcurrentStreams) so it is advertised in the server's
SETTINGS frame.

Signed-off-by: Mekias Yohannes <mmyohannes@gmail.com>

core/dnsserver/config_test.go

@@ -105,6 +105,19 @@ func TestPropagateConfigParamsMaxTCPQueries(t *testing.T) {
 	}
 }
 
+func TestPropagateConfigParamsMaxHTTPSStreams(t *testing.T) {
+	n := 7
+	first := &Config{MaxHTTPSStreams: &n}
+	first.firstConfigInBlock = first
+	second := &Config{firstConfigInBlock: first}
+
+	propagateConfigParams([]*Config{first, second})
+
+	if second.MaxHTTPSStreams == nil || *second.MaxHTTPSStreams != n {
+		t.Fatalf("expected MaxHTTPSStreams to propagate to second config as %d, got %v", n, second.MaxHTTPSStreams)
+	}
+}
+
 func TestPropagateConfigParamsAllowedOpcodes(t *testing.T) {
 	first := &Config{}
 	first.firstConfigInBlock = first