feat: add extra_body to ModelProvider

[nabinchha]

For: #28

• Add extra_body to ModelProvider
• Inject provider spec extra_body to the completion call that gets made. If there's a collision in keys, the one defined in ModelProvider will take precedence over the one in InferenceParameters.
• Change InfoType enum values to lowercase

Testing strategy:

• Existing unit tests pass
• Added new unit tests
• Ran a test workflow locally and verified behavior