feat(cli): add experimental rpty command (#16700)

Relates to https://github.com/coder/coder/issues/16419

Builds upon https://github.com/coder/coder/pull/16638 and adds a command
`exp rpty` that allows you to open a ReconnectingPTY session to an
agent.

This ultimately allows us to add an integration-style CLI test to verify
the functionality added in #16638 .

cli/exp.go

@@ -14,6 +14,7 @@ func (r *RootCmd) expCmd() *serpent.Command {
 			r.scaletestCmd(),
 			r.errorExample(),
 			r.promptExample(),
+			r.rptyCommand(),
 		},
 	}
 	return cmd