Provide a callback mechanism to customize 3p iframe configuration in custom iframe

[cramforce]

Basically people want to do custom things in their "remote.html" file and they should be able to, but ideally with minimal API surface, so they don't hard code assumption into our interface between the frames.

Currently there is code like this

<div id="c" style="position:absolute;top:0;left:0;bottom:0;right:0;">
  <script>draw3p()</script>
</div>

We should allow passing a callback like

<div id="c" style="position:absolute;top:0;left:0;bottom:0;right:0;">
  <script>draw3p(function(config, done) { doSomethingWithConfig(config); done(config) })</script>
</div>

This could e.g. be added to add more targeting info or to switch to a different ad provider.

[cramforce]

CC @kashyapnitin