Add integration tests to detect issues with long-polling push behind reverse proxy

[mcollovati]

There are several tickets in the repository reporting issues when using push with long-polling transport behind a reverse-proxy.
Usually, issues are about messages pushed by the server that for some reason do not reach the client.
Rarely there the reverse also happens, messages sent by the client are not received by the server.

The effect of loosing messages is a forced resynchronization request, that reloads the browser page.
Some user also reported the UI being stuck after the page reload.

These kinds of issues are easy to reproduce, but have been detected, for example after a server reload when Flow client attempts a PUSH reconnection, or after having the laptop resumed from suspension.
In addition, analyzing the reported issues, it seems not all browsers are affected the same way.

To identify and resolve these issues, we can try to set up a test suite to reproduce the problem in the described failing environments.

Below, a couple of related issues:

• java.lang.UnsupportedOperationException: Unexpected message id from the client #16664
• Mobile client idles after a short timeout. #14803
• First request to vaadin application fails #13660
• Vaadin 23 Resynchronizing UI by client's request - If you are using push with a proxy, make sure the push timeout smaller proxy connection timeout #15205

Here is a repo with a couple of reverse-proxy configuration examples: https://github.com/mcollovati/vaadin-reverse-proxy-tests/