[formrecognizer] support `appearance` of a FormLine

v2.1-preview.2 feature:
https://github.com/Azure/azure-rest-api-specs/blob/e590a9714bd94a79cf1f244c42836ae193ea0715/specification/cognitiveservices/data-plane/FormRecognizer/preview/v2.1-preview.2/FormRecognizer.json#L1642

Added, but service is not returning anything for this value yet. Need to update tests/samples when `appearance` is returned.