Create a crate that encapsulates initramfs/init or /sbin/init logic

[bgogul]

Currently, it is duplicated in multiple places:

• https://github.com/project-oak/oak/blob/main/oak_echo_linux_init/src/init.rs
• https://github.com/project-oak/oak/blob/main/oak_hello_world_linux_init/src/init.rs
• https://github.com/project-oak/oak/blob/main/oak_docker_linux_init/src/init.rs
• https://github.com/project-oak/oak/blob/main/oak_minirootfs_linux_init/init

The logic in the above implementations are quite minimal. We should investigate what the responsibilities of the init process and implement them in the crate. e.g., cleaning stale child processes.