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

Current design takes the original GET request parameters, along with a new imp parameter (detailed below), and combines them into a single POST request with the following structure:\

{
   _original GET parameters_\
   params:  {   
               wrapper_type:
               wrapper_vendor:
                    ...
            }
   _array of all adUnits basic data_ 
   imp:     {  
               id:
               h:
               w: 
                    ...
            }
}```

## Phase 1
### General structure change
- [x] Add banner type to supported media type in spec
- [x] Update endpoint to the new hb-multi
- [x] Create the new imp object to send
- [x] 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
- [x] Call endpoint only once for all adUnits, and not per-request as it is now (adapter was built for a single video request)

### function specific 
- [x] `isBidRequestValid` - changed to support multiple bids
- [x] `isBidRequestValid` - check if a different validation is needed compared to video
- [ ] `buildRequests` - check if a banner is built differently from video
- [x] `buildRequests` - build an alternative for `buildVideoRequest` to receive and handle multiple adUnits, and return combined requests object

## Phase 2
- [ ] Create a test object for the test endpoint