feat: add cli command scaletest dynamic-parameters (#20034)

part of https://github.com/coder/internal/issues/912

Adds CLI command `coder exp scaletest dynamic-parameters`

I've left out the configuration of tracing and timeouts for now. I think I want to do some refactoring of the scaletest CLI to make handling those flags take up less boiler plate.

I will add tracing and timeout flags in a follow up PR.

codersdk/client.go

@@ -519,6 +519,16 @@ func (e *Error) Error() string {
 	return builder.String()
 }
 
+// NewTestError is a helper function to create a Error, setting the internal fields. It's generally only useful for
+// testing.
+func NewTestError(statusCode int, method string, u string) *Error {
+	return &Error{
+		statusCode: statusCode,
+		method:     method,
+		url:        u,
+	}
+}
+
 type closeFunc func() error
 
 func (c closeFunc) Close() error {