feat(agent): wire up agentssh server to allow exec into container (#16638)

Builds on top of https://github.com/coder/coder/pull/16623/ and wires up
the ReconnectingPTY server. This does nothing to wire up the web
terminal yet but the added test demonstrates the functionality working.

Other changes:
* Refactors and moves the `SystemEnvInfo` interface to the
`agent/usershell` package to address follow-up from
https://github.com/coder/coder/pull/16623#discussion_r1967580249
* Marks `usershellinfo.Get` as deprecated. Consumers should use the
`EnvInfoer` interface instead.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>
Co-authored-by: Danny Kopping <danny@coder.com>

cli/agent.go

@@ -351,6 +351,8 @@ func (r *RootCmd) workspaceAgent() *serpent.Command {
 				BlockFileTransfer:  blockFileTransfer,
 				Execer:             execer,
 				ContainerLister:    containerLister,
+
+				ExperimentalContainersEnabled: devcontainersEnabled,
 			})
 
 			promHandler := agent.PrometheusMetricsHandler(prometheusRegistry, logger)