Add annotation for specifying backend-service timeout

[evanlucas]

The backend-services created in gke when using this seem to reset the timeout back to 30 under certain circumstances (which I haven't yet figured out). Any chance an annotation could be provided that would allow this to be set? For context, we are using these for websockets, so the default of 30 seconds is not really desirable. I'd be happy to make the changes if this is something yall would be interested in allowing, although, I'm not sure what the preferred annotation name would be.

Thanks!

[evanlucas]

I could have sworn I searched previous issues before posting, but I guess not. Appears to be a duplicate of #28.