completion params instead of model

[shreymodi1]

Add support for arbitrary completion params in RemoteRolloutProcessor [FIR-5739]

Main logic change -

1. Removed model in InitRequest
2. RemoteRolloutProcessor now passes completion params. Row-level overrides config-level completion params.
3. All test servers now retrieve model from completion params
4. Changed typescript servers similarly

[xzrderek]

nice! let dylan / benny take a look and they should be the approver

[xzrderek]

fire

[dphuang2]

lgtm