Fix EZP-22049: As a developer, I want to be able to configure IndexPage

The goal here was to be able to change the page shown as index page, and this should works with semantic configuration too. We made that with a kernel.request event listener which will check if an 'index_page' parameter is defined, and if yes will go forward this page.


- [x] Added a way to configure IndexPage ( / ), to show any other page instead.
- [x] Unit tests.
- [x] Semantic configuration.

link : https://jira.ez.no/browse/EZP-22049

PS : This pull request is mostly the same that ' https://github.com/ezsystems/ezpublish-kernel/pull/657 ' which was closed due to mishandling of my previous branch, sorry !