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>

agent/usershell/usershell.go

@@ -0,0 +1,66 @@
+package usershell
+
+import (
+	"os"
+	"os/user"
+
+	"golang.org/x/xerrors"
+)
+
+// HomeDir returns the home directory of the current user, giving
+// priority to the $HOME environment variable.
+// Deprecated: use EnvInfoer.HomeDir() instead.
+func HomeDir() (string, error) {
+	// First we check the environment.
+	homedir, err := os.UserHomeDir()
+	if err == nil {
+		return homedir, nil
+	}
+
+	// As a fallback, we try the user information.
+	u, err := user.Current()
+	if err != nil {
+		return "", xerrors.Errorf("current user: %w", err)
+	}
+	return u.HomeDir, nil
+}
+
+// EnvInfoer encapsulates external information about the environment.
+type EnvInfoer interface {
+	// User returns the current user.
+	User() (*user.User, error)
+	// Environ returns the environment variables of the current process.
+	Environ() []string
+	// HomeDir returns the home directory of the current user.
+	HomeDir() (string, error)
+	// Shell returns the shell of the given user.
+	Shell(username string) (string, error)
+	// ModifyCommand modifies the command and arguments before execution based on
+	// the environment. This is useful for executing a command inside a container.
+	// In the default case, the command and arguments are returned unchanged.
+	ModifyCommand(name string, args ...string) (string, []string)
+}
+
+// SystemEnvInfo encapsulates the information about the environment
+// just using the default Go implementations.
+type SystemEnvInfo struct{}
+
+func (SystemEnvInfo) User() (*user.User, error) {
+	return user.Current()
+}
+
+func (SystemEnvInfo) Environ() []string {
+	return os.Environ()
+}
+
+func (SystemEnvInfo) HomeDir() (string, error) {
+	return HomeDir()
+}
+
+func (SystemEnvInfo) Shell(username string) (string, error) {
+	return Get(username)
+}
+
+func (SystemEnvInfo) ModifyCommand(name string, args ...string) (string, []string) {
+	return name, args
+}