Add API for Fetching PDF and PPTX Export URLs

Add an api to fetch the export urls pdfs and powerpoints. the view in gamma url worked: https://gamma.app/docs/The-Story-of-Dany-Fawaz-kge3dh2sl9ywbaf?mode=doc the export url structure should be as follows: PDF: https://assets.api.gamma.app/export/pdf/kge3dh2sl9ywbaf/a24ec4b9774ba824097d14c2d1813ee2/The-Story-of-Dany-Fawaz.pdf PPTX: https://assets.api.gamma.app/export/pptx/kge3dh2sl9ywbaf/9a8356e26b0f414264fbf9fa310b3dbd/The-Story-of-Dany-Fawaz.pptx but we only get the item id from the url we receive from the generate api upon successful generation.