[ECS][Request] allow setting runtime for agent

[arkadiyt]

Summary

I'd like to run my ECS task containers on top of gVisor. It seems that this was possible in the past on ECS using an approach like this one:
https://aarongorka.com/blog/gvisor-on-ecs/

Notably the author had to disable the init system and ran the ecs-agent container through docker directly, and also had to manually set some volume binds and other settings. I tried to follow the same approach (updating to include new binds since the post was written, add the NET_ADMIN/SYS_ADMIN capabilities, etc), but in the end it still fails because ecs-agent added an explicit check for not being run directly:
https://github.com/aws/amazon-ecs-agent/blob/7fb101189ae57170a2cc2976d086bec5a4146b97/agent/app/agent_unix.go#L65-L73

It would be great if amazon-ecs-init allowed specifying the runtime so I could run task containers on gVisor and let ecs-agent continue to use runc

[fierlion]

#1072 <- a similar request which originated in ecs-agent repo