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.

Start by locating the Java context parameter API and the existing environment-variable access path described in the issue. Trace how the host supplies the context at runtime, then identify the unit-test coverage needed to verify that a mocked context can provide a setting while runtime access still reads the environment variable.

Written by the indexing model from the issue text.