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: 
                    ...
            }
}

Original Get request parameters:

{ 
    wrapper_type: 
    wrapper_vendor: 
    wrapper_version: 
    adapter_version:
    auction_start: 
    ad_unit_code: 
    tmax:,
    width:,
    height:,
    publisher_id:
    floor_price: 
    ua: 
    bid_id: 
    bidder_request_id: 
    transaction_id:
    session_id:
    publisher_name: 
    site_domain:
    dnt:
    device_type:
userIds,
site_metadata,
user_metadata,
playback_method,placement,pos,
min_duration,
max_duration,
skip,
linearity,
placement_id,
cs_method,
us_privacy,
gdpr,
gdpr_consent,
ifa,
schain,
referrer,
page_url



## 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