feat: change backend functions to be async

[jakelorocco]

Changes

• made generate_from_context (and functions called by it) async
• made generate_from_raw async
• added generate_from_raw support to litellm
• fixed all invocations of these functions to await the results
• added stricter tests to ensure values are populated and expected results / errors happen

Tests:

• all tests pass locally
• all examples run

[mergify]

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