Add support for RTC Config

[pradeep910]

Many sites use Programmatic header bidding like prebid.js etc. that callouts multiple ad request and fetches high paying ads by doing quick auction.

AMP RTC Config enables us to use header bidding -
https://github.com/ampproject/amphtml/blob/master/extensions/amp-a4a/rtc-documentation.md

Some references -
https://medium.com/ampfuel/better-than-header-bidding-amp-rtc-fc54e80f3999
http://prebid.org/formats/amp.html

[sagarnasit]

@pradeep910 We don't have any vendors with we can test RTC config. Let me know how to progress further with this feature implementation.

[pradeep910]

Lets block it for now. We will take it up later.