feat: Add workspace agent for SSH (#318)

* feat: Add workspace agent for SSH

This adds the initial agent that supports TTY
and execution over SSH. It functions across MacOS,
Windows, and Linux.

This does not handle the coderd interaction yet,
but does setup a simple path forward.

* Fix pty tests on Windows

* Fix log race

* Lock around dial error to fix log output

* Fix context return early

* fix: Leaking yamux session after HTTP handler is closed

Closes #317. We depended on the context canceling the yamux connection,
but this isn't a sync operation. Explicitly calling close ensures the
handler waits for yamux to complete before exit.

* Lock around close return

* Force failure with log

* Fix failed handler

* Upgrade dep

* Fix defer inside loops

* Fix context cancel for HTTP requests

* Fix resize

go.mod

@@ -20,6 +20,7 @@ require (
 	github.com/coder/retry v1.3.0
 	github.com/creack/pty v1.1.17
 	github.com/fatih/color v1.13.0
+	github.com/gliderlabs/ssh v0.3.3
 	github.com/go-chi/chi/v5 v5.0.7
 	github.com/go-chi/render v1.0.1
 	github.com/go-playground/validator/v10 v10.10.0
@@ -64,6 +65,7 @@ require (
 	github.com/Nvveen/Gotty v0.0.0-20120604004816-cd527374f1e5 // indirect
 	github.com/agext/levenshtein v1.2.3 // indirect
 	github.com/alecthomas/chroma v0.10.0 // indirect
+	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be // indirect
 	github.com/apparentlymart/go-textseg/v13 v13.0.0 // indirect
 	github.com/cenkalti/backoff/v4 v4.1.2 // indirect
 	github.com/chzyer/readline v0.0.0-20180603132655-2972be24d48e // indirect