Remove the `df` parameter from orchestration contexts

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.

Start by tracing the orchestration APIs exposed through the Context object's df property, including callActivity(), callSubOrchestration(), and currentUtcDateTime. Review how Context extends or receives Azure Functions metadata. Done requires deciding whether to flatten these APIs or introduce a dedicated orchestration context type, then documenting the selected API shape.

Written by the indexing model from the issue text.