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/register.go

@@ -296,6 +296,10 @@ func propagateConfigParams(configs []*Config) {
 		// Propagate UDPDecorateWriterFunc so a decorator configured once in a
 		// server block applies to the block's UDP listener(s).
 		c.UDPDecorateWriterFunc = c.firstConfigInBlock.UDPDecorateWriterFunc
+
+		// Propagate MaxHTTPSStreams so a `https { max_streams N }` set once in a
+		// server block applies to the block's HTTPS key regardless of key order.
+		c.MaxHTTPSStreams = c.firstConfigInBlock.MaxHTTPSStreams
 	}
 }