Add the ability for scheduling plugins to add/modify requests during request processing

[shmuelk]

What would you like to be added:
Add the ability for PreSchedule, Filter, Scorer, Picker, and PostSchedule scheduler plugins to add/modify the set of headers that are sent to the chosen inference pod with the request.

Why is this needed:
Some implementations of disaggregated Prefill/Decode processing require the use of additional headers in the request that is sent to the inference server.

If acceptable, I am happy to contribute the code.

[shmuelk]

/assign