Convert SamplingParams.strategy to a union

[hardikjshah]

What does this PR do?

Cleans up how we provide sampling params. Earlier, strategy was an enum and all params (top_p, temperature, top_k) across all strategies were grouped. We now have a strategy union object with each strategy (greedy, top_p, top_k) having its corresponding params.
Earlier,

class SamplingParams: 
    strategy: enum ()
    top_p, temperature, top_k and other params

However, the strategy field was not being used in any providers making it confusing to know the exact sampling behavior purely based on the params since you could pass temperature, top_p, top_k and how the provider would interpret those would not be clear.

Hence we introduced -- a union where the strategy and relevant params are all clubbed together to avoid this confusion.

Have updated all providers, tests, notebooks, readme and otehr places where sampling params was being used to use the new format.

Test Plan

pytest llama_stack/providers/tests/inference/groq/test_groq_utils.py
// inference on ollama, fireworks and together
with-proxy pytest -v -s -k "ollama" --inference-model="meta-llama/Llama-3.1-8B-Instruct" llama_stack/providers/tests/inference/test_text_inference.py
// agents on fireworks
pytest -v -s -k 'fireworks and create_agent' --inference-model="meta-llama/Llama-3.1-8B-Instruct" llama_stack/providers/tests/agents/test_agents.py --safety-shield="meta-llama/Llama-Guard-3-8B"

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.

[ashwinb]

looks good to me! thank you for going through all the providers....