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/workspacesdk.go

@@ -12,12 +12,14 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/google/uuid"
-	"golang.org/x/xerrors"
 	"tailscale.com/tailcfg"
 	"tailscale.com/wgengine/capture"
 
+	"github.com/google/uuid"
+	"golang.org/x/xerrors"
+
 	"cdr.dev/slog"
+
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/tailnet"
 	"github.com/coder/coder/v2/tailnet/proto"
@@ -305,6 +307,16 @@ type WorkspaceAgentReconnectingPTYOpts struct {
 	// issue-reconnecting-pty-signed-token endpoint. If set, the session token
 	// on the client will not be sent.
 	SignedToken string
+
+	// Experimental: Container, if set, will attempt to exec into a running container
+	// visible to the agent. This should be a unique container ID
+	// (implementation-dependent).
+	// ContainerUser is the user as which to exec into the container.
+	// NOTE: This feature is currently experimental and is currently "opt-in".
+	// In order to use this feature, the agent must have the environment variable
+	// CODER_AGENT_DEVCONTAINERS_ENABLE set to "true".
+	Container     string
+	ContainerUser string
 }
 
 // AgentReconnectingPTY spawns a PTY that reconnects using the token provided.
@@ -320,6 +332,12 @@ func (c *Client) AgentReconnectingPTY(ctx context.Context, opts WorkspaceAgentRe
 	q.Set("width", strconv.Itoa(int(opts.Width)))
 	q.Set("height", strconv.Itoa(int(opts.Height)))
 	q.Set("command", opts.Command)
+	if opts.Container != "" {
+		q.Set("container", opts.Container)
+	}
+	if opts.ContainerUser != "" {
+		q.Set("container_user", opts.ContainerUser)
+	}
 	// If we're using a signed token, set the query parameter.
 	if opts.SignedToken != "" {
 		q.Set(codersdk.SignedAppTokenQueryParameter, opts.SignedToken)