Support selection marks

[maririos]

• As type on FormField
• As a list[SelectionMarks] on FormPage
• As an element type on FieldData/FormTableCell - requires parsing the json pointer
• tests
• samples
• documentation
• mocking

[maririos]

• Available only for supervised only models.
• Selected, unselected are the only values returned on SelectionMark.State. In the future undetermined could be there too. No more. It is ok to treat them as enum as service wants to add modelAsString=false in the swagger file
• Looks like for 2.1-preview.2 the SelectionMarks will be included as an json pointer under elements (need to confirm)

[maririos]

Design:
https://apiview.dev/Assemblies/Review/cf2c57b3d4c44ef08c2e8284f7441a2d?diffRevisionId=714267e3f977493fa2690daf64c2df11

Sample: https://github.com/Azure/azure-sdk-for-net/blob/feature/formrecognizer2.1/sdk/formrecognizer/Azure.AI.FormRecognizer/samples/Sample1_RecognizeFormContent.md#recognize-form-content-from-a-uri

[maririos]

Looks like for 2.1-preview.2 the SelectionMarks will be included as an json pointer under elements (need to confirm)

Service made an update and it is now included in 2.1-preview.1. format is i.e. #/readResults/0/selectionMarks/2

[maririos]

Info for documentation -> https://docs.microsoft.com/en-us/azure/cognitive-services/form-recognizer/whats-new#new-features