Add alternatebiddercodes in Prebid request extension

[pm-nilesh-chate]

w.r.t prebid#2174 (comment)

This changes would make alternatebiddercodes from PBS config formally part of Prebid ORTB extensions.

• Incoming ORTB would contain:

ext.prebid.alternatebiddercodes.enabled: true/false
ext.prebid.alternatebiddercodes.bidders.BIDDER.enabled: true/false
ext.prebid.alternatebiddercodes.bidders.BIDDER.allowedbiddercodes: [array, of, strings]

• This would override the account-level config
• If the request doesn't specify these values, then PBS-core would merge the account config into these values
• Before sending to each adapter, PBS-core would need to remove entries that don't belong to that bidder. i.e. they would only receive ext.prebid.alternatebiddercodes.adapters.ME
• Everything else is up to the adapter. They can ignore or use at will.

Ex. s2s config on page and incoming PBS request

pbjs.setConfig({
    s2sConfig: {
        extPrebid: {
            "alternatebiddercodes": {
                "enabled": true,
                "bidders": {
                    "pubmatic": {
                        "enabled": true,
                        "allowedbiddercodes": [
                            "groupm"
                        ]
                    }
                }
            }
        }
    }
});

{
   ...
   "ext": {
       "prebid": {
           "alternatebiddercodes": {
                "enabled": true,
                "bidders": {
                    "pubmatic": {
                        "enabled": true,
                        "allowedbiddercodes": [
                            "groupm"
                        ]
                    }
                }
            }
       }
   }
}

More details at prebid#2174

[pm-nilesh-chate]

resolved conflict and raised pr with vanilla base prebid#2339