feat: graceful termination of the init process

[clementrey-dev]

Dear maintainers

This pull request is a proposed implementation for the graceful termination of the init process through two modifications

• a timeout named a graceful termination that corresponds to the time, starting when the SIGTERM is received, after which the
  init process is over
• a new HTTP endpoint (/runtime/stop) for shutting down properly the init process

Thank you

[steve-a-jones]

Have a use case for this as well since I need to test proper handling of sigterm traps for my runtime. Thanks for putting the work in for this.