[BUG] function_schema_parser does not support new python union types

[JaredDobry]

Similar to #50
You can't do this:

@dataclass
class ExternalImageSearchResponse:
    result: None | external.ImageSearchResponse
    error: None | str

You must do this:

@dataclass
class ExternalImageSearchResponse:
    result: Optional[external.ImageSearchResponse]
    error: Optional[str]

or the parser throws an exception

[scottkruyswyk]

Only some_type | None should be supported; we should also throw a clear error message if some other union type is used