[6.x] Update plugin lifecycle

[riasvdv]

Description

Moves plugin lifecycle ownership from Laravel package discovery to the Plugins service.

A few key changes:

• Plugins can now use normal register and boot methods like any normal Laravel service provider.
• Plugins should no longer add the class to composer.json's extra.laravel.providers key and only in extra.class (Craft plugin convention) since we own when the plugin service provider gets registered and booted. We guard against this with a self-explanatory exception. Any additional service providers that need to run outside the enabled/installed detection can still be added there.
• The automatic trait registration system has been removed for a fixed method which calls the trait methods at the right time. Much simpler to understand and doesn't cause any unexpected side-effects.
• Publishes plugin's assets when we publish Craft's assets with craft:setup:publish

Important

This is breaking for any plugins that are currently targeting 6.x, but this is alpha so should not be unexpected.

Upgrade guide:

• Rename registerPlugin to register in your plugin class
• Rename bootPlugin to boot in your plugin class
• Remove the plugin class from extra.laravel.providers in composer.json

[github-actions]

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