Remove the `df` parameter from orchestration contexts

[hossam-nasr]

Currently, all orchestration APIs are provided under a df parameter inside the Context object, e.g., context.df.callActivity(), context.df.callSubOrchestration(), context.df.currentUtcDateTime, etc.

Since all these durable APIs are a lot more relevant to the user than the other metadata (coming from Azure Functions directly) present in the context object, this issue is to explore whether we should provide the APIs directly in the Context object rather than nested inside a df property. We could also define our own type of orchestration context, picking only the relevant elements from the Functions context, instead of extending it.

[hossam-nasr]

There are 3 options I can see here:

1. Keep it as-is:

Durable APIs live under a df object, and OrchestrationContext/EntityContext types extend InvocationContext

2. Spread the Durable APIs in the main object:

Durable contexts (OrchestrationContext/EntityContext) still extend InvocationContext, but the Durable APIs live directly on the context object, rather than under one df object.

3. Flip the order:

Durable contexts contain the Durable APIs directly, but then add an invocationContext parameter that contains the InvocationContext object. In that way, Durable contexts would behave similarly to execution hook contexts.

[davidmrdavid]

@hossam-nasr: From my understanding, we decided to keep this as-is for now. Is that correct?

[hossam-nasr]

@davidmrdavid Yes, I believe we decided to skip this for preview. Removed the preview label