feat(speculative-sampling): allow to specify a draft model in the model config

[mudler]

Description

This PR fixes #1013.

It adds draft_model and n_draft to the model YAML config in order to load models with speculative sampling. This should be compatible as well with grammars.

example:

backend: llama                                                                                                                                                                   
context_size: 1024                                                                                                                                                                        
name: my-model-name
parameters:
  model: foo-bar
n_draft: 16                                                                                                                                                                      
draft_model: model-name