Extract DrupalKernel::bootEnvironment into SAPI adapter

Closing to focus on common solution ✨ Use symfony/runtime for less bespoke bootstrap/compatibility with varied runtime environments Active

Reopening this but postponing on ✨ Use symfony/runtime for less bespoke bootstrap/compatibility with varied runtime environments Active so that that issue can remain small and focused on adding the runtime to Drupal but this issue moves over the actual environment set-up.