[Epic] Refactor generate_data.py to make it functional and simpler

[shivchander]

• Num_instructions/sdg_scale_factor currently is setup only for the freeform skills - we need some logic to split that value between contexts and grounded questions for grounded skills. https://github.com/instructlab/sdg/blob/main/src/instructlab/sdg/generate_data.py#L189
• The current state of this code doesn't generate grounded data - it treates them as freeform (Grounded flow missing num_samples #99, Update generate_data.py to capture context key #98, grounded flow not capturing context #97)
• We should move the sys_prompt and system roles and delineate that to data mixing step [EPIC] Data mixing #95
• Let go of legacy keys used for self instruct Let go of legacy simple SDG self instruct leaf node keys #102

[markmc]

I'm not sure this epic captures anything that's still actionable. If there is something actionable not captured in an individual issue, please file an issue for that. Thanks 👍