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>

codersdk/workspacesdk/agentconn.go

@@ -93,6 +93,24 @@ type AgentReconnectingPTYInit struct {
 	Height  uint16
 	Width   uint16
 	Command string
+	// Container, if set, will attempt to exec into a running container visible to the agent.
+	// This should be a unique container ID (implementation-dependent).
+	Container string
+	// ContainerUser, if set, will set the target user when execing into a container.
+	// This can be a username or UID, depending on the underlying implementation.
+	// This is ignored if Container is not set.
+	ContainerUser string
+}
+
+// AgentReconnectingPTYInitOption is a functional option for AgentReconnectingPTYInit.
+type AgentReconnectingPTYInitOption func(*AgentReconnectingPTYInit)
+
+// AgentReconnectingPTYInitWithContainer sets the container and container user for the reconnecting PTY session.
+func AgentReconnectingPTYInitWithContainer(container, containerUser string) AgentReconnectingPTYInitOption {
+	return func(init *AgentReconnectingPTYInit) {
+		init.Container = container
+		init.ContainerUser = containerUser
+	}
 }
 
 // ReconnectingPTYRequest is sent from the client to the server
@@ -107,7 +125,7 @@ type ReconnectingPTYRequest struct {
 // ReconnectingPTY spawns a new reconnecting terminal session.
 // `ReconnectingPTYRequest` should be JSON marshaled and written to the returned net.Conn.
 // Raw terminal output will be read from the returned net.Conn.
-func (c *AgentConn) ReconnectingPTY(ctx context.Context, id uuid.UUID, height, width uint16, command string) (net.Conn, error) {
+func (c *AgentConn) ReconnectingPTY(ctx context.Context, id uuid.UUID, height, width uint16, command string, initOpts ...AgentReconnectingPTYInitOption) (net.Conn, error) {
 	ctx, span := tracing.StartSpan(ctx)
 	defer span.End()
 
@@ -119,12 +137,16 @@ func (c *AgentConn) ReconnectingPTY(ctx context.Context, id uuid.UUID, height, w
 	if err != nil {
 		return nil, err
 	}
-	data, err := json.Marshal(AgentReconnectingPTYInit{
+	rptyInit := AgentReconnectingPTYInit{
 		ID:      id,
 		Height:  height,
 		Width:   width,
 		Command: command,
-	})
+	}
+	for _, o := range initOpts {
+		o(&rptyInit)
+	}
+	data, err := json.Marshal(rptyInit)
 	if err != nil {
 		_ = conn.Close()
 		return nil, err