Allow directly setting `OpenApiGeneratorTask.waitTimeInSeconds`

[westse]

waitTimeInSeconds is currently the only private property. Making it public will allow directly setting it by other plugins.

I need this currently to workaround issues like this one which cause tasks, including OpenApiGeneratorTask, to be configured too early such that build.gradle values are not bound yet to OpenApiExtension. Explicitly setting each property works for all but waitTimeInSeconds because it is private.

I suspect my root issue is #117 and its fix (perhaps #124 ) will help. However, this issue seems still relevant.