feat(cli): add trafficgen command for load testing (#7307)

This PR adds a scaletest workspace-traffic command for load testing. This opens a
ReconnectingPTY connection to each scaletest workspace (via coderd) and 
concurrently writes and reads random data to/from the PTY. Payloads are of the
form #${RANDOM_ALPHANUMERIC_STRING}, which essentially drops garbage
comments in the remote shell, and should not result in any commands being executed.

codersdk/workspaceagentconn.go

@@ -165,9 +165,9 @@ type WorkspaceAgentReconnectingPTYInit struct {
 // to pipe data to a PTY.
 // @typescript-ignore ReconnectingPTYRequest
 type ReconnectingPTYRequest struct {
-	Data   string `json:"data"`
-	Height uint16 `json:"height"`
-	Width  uint16 `json:"width"`
+	Data   string `json:"data,omitempty"`
+	Height uint16 `json:"height,omitempty"`
+	Width  uint16 `json:"width,omitempty"`
 }
 
 // ReconnectingPTY spawns a new reconnecting terminal session.