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_windows.go

@@ -3,6 +3,7 @@ package usershell
 import "os/exec"
 
 // Get returns the command prompt binary name.
+// Deprecated: use SystemEnvInfo.UserShell instead.
 func Get(username string) (string, error) {
 	_, err := exec.LookPath("pwsh.exe")
 	if err == nil {