Core: support for expandable / flex slots

[dgirardi]

Type of change

• Feature

Description of change

This adds better support for banner slots with flexible sizes:

• publishers can now specify an ortb format array instead of sizes for an ad unit's mediaTypes.banner; expdir can also be supplied there for convenience (alias of ortb2Imp.banner.expdir);
• bid adapters can reply with wratio/hratio instead of width and height;
• because many adapters expect specific sizes, format elements that only specify wratio/hratio are translated to "placeholder" sizes (e.g. {wratio: 2, hratio: 1} becomes [2, 1]);
• creatives now attempt to expand to all the available width/height when rendering a bid that has no definite size.

Other information

#9361