add exec support

[drewbailey]

Creating a Draft PR containing WIP for basic exec support.

I don't think we'll easily be able to support TTY without significant work in handling / upgrading connections to websockets, we may be able to shim the docker go api package to use their exec command against a podman endpoint

[towe75]

I checked some code and found that both websocket and raw tcp can be used:

• cockpit-podman has a http based remote terminal using a websocket: https://github.com/cockpit-project/cockpit-podman/blob/master/src/ContainerTerminal.jsx
• podmans own binding package upgrades the http call to a raw tcp connection. Here we would get distinct stderr/stdout streams but handling is somewhat complex: https://github.com/containers/podman/blob/v2.1/pkg/bindings/containers/attach.go

Maybe we can borrow/share some code from there?
Documentation is rather poor, i could not find any details whatsoever.

[towe75]

@drewbailey could not resist to give it a try. Please take a look at my branch 55d4420

I ported podmans own attach.go to fit into our client structure. I've got a working interactive cli (nomad alloc exec) and web-shell, also scripted consul health checks are and using e.g. pipelines in a shell is also ok.

For sure it needs some polishing and something is still wrong with terminal/window resizing - although i did the necessary plumbing and neither nomad nor podman complain about something.

I am also not sure if stderr forwarding works correctly in interactive mode, never tried the consul client so i don't know how it should behave.

It's certainly a very good starting point and it might even be enough for master.
Be aware that it must be adopted to my actor branch later.

[towe75]

Resize problem seems to be a podman bug, see containers/podman#7102

[towe75]

@drewbailey : can we close this one in favor of #87 ?