Provide access to settings through context parameter

I created the following issue on the java worker repo, but this repo seems to be a better fit. https://github.com/Azure/azure-functions-java-worker/issues/281

Currently it is hard to unit test a function that uses an app setting by reading it from the environment variables. It would be much easier if there was a getSetting(settingName) method on the context parameter, as the context parameter can be mocked in unit tests. At runtime the host can continue to read the value from an environment variable via the method in the context parameter, so there wouldn't be any breaking changes to the runtime.