Improve Digital Bid adapter: use the oRTB server endpoint (prebid#8138)

* Major refactoring to use new oRTB server endpoint

Co-authored-by: Faisal Islam <100519197+iosfaisal@users.noreply.github.com>
Co-authored-by: Jozef Bartek <31618107+jbartek25@users.noreply.github.com>

* fixed wrong merge

* Fixed issue related to site and app (#9)

* Fixed errors for test cases.

Co-authored-by: Faisal Islam <93644923+faisalvs@users.noreply.github.com>
Co-authored-by: Faisal Islam <100519197+iosfaisal@users.noreply.github.com>
Co-authored-by: Faisal Islam <faisal.islam@vivacomsolutions.com>