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.

首先定位 Java 上下文参数 API 以及 issue 中描述的现有环境变量访问路径。跟踪 host 在运行时如何提供上下文，然后确定所需的单元测试覆盖范围，以验证 mocked context 可以提供一个设置，而运行时访问仍然读取环境变量。

由索引模型根据 Issue 内容生成。