feat: restrict mime types on Image IO descriptor

[ssheng]

Allow only a subset of image mime types and reject the rest.

@api(input=Image(mime_type=['image/jpeg', 'image/png'], validate=True), output=JSON())
def predict(input_image: PIL.Image.Image) -> dict:
  ...

[ssheng]

Potentially extend the validation to all IO descriptors for mime_types.