feat: change the semantics of function calls to put the returned value in pdl_context

[mandel]

With this PR, the background context of the body of a function stays local to the function and only the result of the function is added to the context when it is called.

For example, in the following program:

defs:
  f:
    function:
    return:
      lastOf:
      - How are you?
      - Bye
lastOf:
- Hello
- call: ${f}
- ${pdl_context}

The output is

[ {'role': 'user', 'content': 'Hello'},
  {'role': 'user', 'content': 'Bye'} ]

Warning: inlining a function might change the background context.