shell: kernel_service: reorg the commands

Split the `kernel_service.c` into multiple subcommand files,
each file would register with the main `kernel` cmd based on
the dependencies in Kconfig/CMakeLists.txt.

This greatly reduces the number of precompiler directives.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>