Increase default firewall strategy for plugin legacy scripts

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

With the proposed changes, the default strategy applied to plugins routes/scripts will be similar than the default strategy applied to the core GLPI routes/scripts:

• for legacy scripts, the firewall will require a valid session,
• for Symfony routes, the firewall will require the user to has access to the central interface.

For the few plugins that would want to specify a specific strategy, it will be possible to do this:

• either by using the method to define a strategy for legacy scripts \Glpi\Http\Firewall::addPluginStrategyForLegacyScripts() (e.g. Firewall::addPluginStrategyForLegacyScripts('myplugin', '#^/front/helpdesk_.*#', Firewall::STRATEGY_HELPDESK_ACCESS););
• either by using the \Glpi\Security\Attribute\SecurityStrategy attribute on the correspondiing symfony route.

This change corresponds to a TODO introduced in #15614.