[Durable Functions] Basic "Function Chaining" implementation

[AnatoliB]

Please note:

• The main focus of this PR is on enabling the "Long-Running HTTP Request" scenario, which requires just a single activity in the chain. As a side effect, the entire "Function Chaining" pattern also works in general. However, this is not the main goal right now.
• To enable the new functionality, the user has to set PSWorkerEnableExperimentalDurableFunctions to "true" (via app settings or local.settings.json).
• The current implementation is incomplete and does not have enough automated test coverage. Implementing the missing pieces and improving test coverage will take some time. However, we want to deploy this implementation as soon as possible in order to validate the "Long-Running HTTP Request" scenario end-to-end. After this is done, we will complete the work incrementally.

[JustinGrote]

Nice work, this opens up a lot of scenarios we currently handle either in Azure Automation or with a script-runner on a box.

[Francisco-Gamino]

A few minor comments. Otherwise, LGTM.