Add a hook for plugins to add HTML code on pages

`Html::includeHeader()` offers hooks for plugins to add CSS and JavaScript assets (via `$PLUGIN_HOOKS['add_css']` and `$PLUGIN_HOOKS['add_javascript']`) but nothing for plain HTML code (static or dynamically generated).

Modifying `Html::footer()` would allow plugins to add code on/as footer.

More method are impacted because some are dedicated to the help pages, modal/pop-up, etc.

I suggest adding hooks (say `$PLUGIN_HOOKS['add_code_in_header']` and `$PLUGIN_HOOKS['add_code_in_footer']`) for the plugin to allow more customization of GLPI general interface.

In my case, I am building a plugin to add snippets of plain HTML codes (disclaimers, notes, analytic trackers, etc.) on any pages.