Make the retry-after value in GenericRandomErrorPlugin configurable

[waldekmastykarz]

When defining throttling responses, you can configure the GenericRandomErrorPlugin to return a 429 response with a Retry-After header. You can configure the header to a static value or to @dynamic. When you configure it do @dynamic, the Retry-After header will be set to the number of seconds remaining until the throttling window reset. Right now, when you use @dynamic, the reset window is fixed to 5 seconds. Ideally, this value should be configurable to allow for more flexibility.

Steps:

• Introduce a new configuration property in the GenericRandomErrorPlugin named RetryAfterInSeconds
• Update docs