add litellm model

[kausmeows]

Description

• Summary of changes: Add a new litellm model provider
• Related issues: Mention if this PR fixes or is connected to any issues.
• Motivation and context: Explain the reason for the changes and the problem they solve.
• Environment or dependencies: Specify any changes in dependencies or environment configurations required for this update.
• Impact on metrics: (If applicable) Describe changes in any metrics or performance benchmarks.

Fixes # (issue)

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Type of change

Please check the options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Model update (Addition or modification of models)
• Other (please describe):

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Checklist

• Adherence to standards: Code complies with Agno’s style guidelines and best practices.
• Formatting and validation: You have run ./scripts/format.sh and ./scripts/validate.sh to ensure code is formatted and linted.
• Self-review completed: A thorough review has been performed by the contributor(s).
• Documentation: Docstrings and comments have been added or updated for any complex logic.
• Examples and guides: Relevant cookbook examples have been included or updated (if applicable).
• Tested in a clean environment: Changes have been tested in a clean environment to confirm expected behavior.
• Tests (optional): Tests have been added or updated to cover any new or changed functionality.

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Additional Notes

Include any deployment notes, performance implications, security considerations, or other relevant information (e.g., screenshots or logs if applicable).

[kausmeows]

Hey @dirkbrnd I've refactored the code and done a cleaner implementation. Can you have a look again??
Will also add tests soon