Support banner

This page will accommodate the new adapter design, to support banner (display) type media, in our adapter

Things to consider

General structure

• add banner type to supported media type
• update endpoint to new hb-multi
• create the new imp object to send
• change GET request to POST (and verify old behavior is not affected and destroyed)
• Find what's common between video and banner request and reuse data when building object\

function specific

• isBidRequestValid - does is need to be changed? is there a different validation for banners?
• build request - what's needed for a banner request compared to video