Feature: Add TextQuoteSelector to Annotation Spec

introduced in subugoe/ahiqar/backend!292 (comment 1201404)

we are utilizing this fine selector to provide information on the text passage that is annotated within the annotation object itself. this enables clients to deal with the annotation standalone, meaning no further resources is needed for some analysis. also it will support the check for correct selectors allowing to compare the target from other selectors to the text provided here.