Add Singleton support for Functions to ensure only one function running at a time

We should discuss whether we want to bring this functionality forward for Functions. However, it will be relatively simple for us to do - we just need to expose new properties via function.json for function level singletons. See [here](https://github.com/Azure/azure-webjobs-sdk/wiki/Singleton) for singleton doc.

In addition to Function level singleton support, we might also consider supporting Listener level Singleton as well.

When we do this, we should make sure it works across multiple languages (not just C#)

Start with the linked Singleton documentation and the function.json properties mentioned in the issue. Clarify the intended function-level and listener-level scope, including behavior across languages, then define the tests and acceptance criteria for ensuring only one function runs at a time.

Written by the indexing model from the issue text.