pass ClientInfo to Transport.ConnContext, allow rejection of handshake (#5122)

* pass *ClientInfo to ConnContext

This allows users to set some state related to the client's remote
address in `ConnContext`.

This also refuses connection early if the returned context is cancelled.

* review comments

* better error assertion

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

integrationtests/self/http_test.go

@@ -614,8 +614,8 @@ func TestHTTPContextFromQUIC(t *testing.T) {
 	conn := newUDPConnLocalhost(t)
 	tr := &quic.Transport{
 		Conn: conn,
-		ConnContext: func(ctx context.Context) context.Context {
-			return context.WithValue(ctx, "foo", "bar") //nolint:staticcheck
+		ConnContext: func(ctx context.Context, _ *quic.ClientInfo) (context.Context, error) {
+			return context.WithValue(ctx, "foo", "bar"), nil
 		},
 	}
 	defer tr.Close()