optionally define a delay in webhook

[nhomble]

A follow up from the discussion on https://github.com/wiremock/wiremock-webhooks-extension/pull/20

This change adds a delay parameter to the webhook definition. Thanks for the feedback @tomakehurst, I also would have preferred java.time.Duration but that would have required java 8+ and wasn't convinced this should be the reason to force the new version. Instead, a delay in milliseconds should reasonably cover any latency you would expect in a test.

[PatrickLaflamme]

This change would solve my problem.

[nhomble]

@tomakehurst any concerns with this?

[tomakehurst]

Can we change it to fixedDelay for consistency with WireMock's stubbing?

Also BTW, I'm thinking of making this extension a subproject of WireMock and starting to release it with WireMock under the same version number. I think this should help avoid version conflicts in dependencies etc. and enable it to be used standalone without as much hassle.