Rubicon Bid Adapter: add native support (prebid#9574)

* add support for native

* wrap native tests around FEATURES.NATIVE

* remove commented out code

* HB-16092 support multiformat parameter

* do not generate imp if has only banner media type

* check banner bid type only if mediaTypes.banner

* new multiformat logic

* bidonmultiformat

* fixes: do not set empty keywords; better behavior for floors.

* currency is always added

* remove prorperties that are already set by ortb

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Co-authored-by: Michele Nasti <michele@rtk.io>