[CustomHandler]Add config option to customize process init time

As of now, once the worker process is launched functions host waits for 30secs for the custom handler to be initialized and ready:

https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script/Workers/Http/DefaultHttpWorkerService.cs#L176

If CustomHandler does not respond within 30secs, process is restarted .

Exposing an config option to customize this timeout will allow users to increase the timeout in cases where they know initialization could take longer.

@anthonychu @yojagad - Let me know if you think this will be useful. If yes, we should set an upperbound.

Start with src/WebJobs.Script/Workers/Http/DefaultHttpWorkerService.cs at line 176 and trace how the 30-second custom-handler initialization timeout is applied. Identify the existing configuration path and determine how an upper bound should be represented. Done means the timeout can be configured for slower custom-handler startup without changing the default behavior.

Written by the indexing model from the issue text.