replace v1/completions with v1/chat/completions

[aseelert]

Currently the legacy v1 API calls are used, so there are a lot of limits in terms of modes and the TPM of them.
https://platform.openai.com/docs/api-reference/completions/create

there should be a check for endpoint https://api.openai.com/v1/completions or https://api.openai.com/v1/chat/completions

as currently the SDG fails with just using the newer type:

packages/instructlab/sdg/pipeline.py", line 203, in _generate_single
    raise PipelineBlockError(
instructlab.sdg.pipeline.PipelineBlockError: 
PipelineBlockError(<class 'instructlab.sdg.llmblock.LLMBlock'>/gen_contexts): Error code: 400 - 
{'error': {'message': 
"Missing required parameter: 'messages'.", 
'type': 'invalid_request_error', 
'param': 'messages',
'code': 'missing_required_parameter'}}

curl https://api.openai.com/v1/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -d '{
    "model": "gpt-3.5-turbo-instruct",
    "prompt": "Say this is a test",
    "max_tokens": 7,
    "temperature": 0
  }'

vs new:

curl https://api.openai.com/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -d '{
     "model": "gpt-4o-mini",
     "messages": [{"role": "user", "content": "Say this is a test!"}],
     "temperature": 0.7
   }'