Support banner

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

Changes are based on the prebid docs at: prebid docs

Phase 1

General structure change

• Add banner type to supported media type in spec
• Update endpoint to the new hb-multi
• Create the new imp object to send
• Change GET request to POST (and verify old behavior is not affected or destroyed)
• Find what's common between video and banner request and reuse data when building object

function specific

• isBidRequestValid - changed to support multiple bids
• isBidRequestValid - check if a different validation is needed compared to video
• buildRequests - check if a banner is built differently from video

Phase 2

• Create a test object for the test endpoint