Update front-controllers to get dependencies from DrupalRuntime

Problem/Motivation

#3313404: Use symfony/runtime for less bespoke bootstrap/compatibility with varied runtime environments added DrupalRuntime but kept the changes to the front-controller minimal. The front-controllers still contain hardcoded inputs to DrupalKernel (e.g. the autoload require and environment). This means that these things still require changes to the front-controllers rather than being able to control these based on the environment.

Steps to reproduce

Proposed resolution

Implement DrupalKernel::getArgument so that we can make the arguments currently hardcoded to DrupalKernel available as argument injections to the main front-controller function.

The project directory is provided by Symfony's autoload_runtime.php file. If it does not exist then we find the project directory based on Composer's ClassLoader location.

Remaining tasks

User interface changes

Introduced terminology

API changes

Front controllers can now inject the following variables into their main application function:

• string $environment - The current environment or prod by default. The environment is read from $_SERVER['APP_ENV'] or $_ENV['APP_ENV'] by default. This behavior can be changed by configuring runtime options
• Any variable type-hinted with ClassLoader will contain the composer autoloader as if running require vendor/autoload.php.

Data model changes

Release notes snippet