Support for Native in Pubmatic Adapter (prebid#3408)

* changes to support native in pubmaticbid adapter

* Removed port from endpoint

* Removed protocol from endpoint

* Formatting

* Fix request payload

* Updated test case

* Changed request and response as per ortb spec

* Change in request and response

* Removed comments and extra code

* Code Review comments

* Code Review Comments and Test cases for request and response

* Removed data type as all data asset types are handled

* Code Review Changes

* Code Review Comments

* Supporting both banner and native and sending 0x0 in case of native

* Bug Fixes

* Bug response not processed by prebid

* Change warning message

* Fixed typo

* Do not send request in case of invalid native bid

* Do not send request in case of invalid native requests

* objects converted to strings in log for debug purposes

* Fixed logic to check for required parmas

* Fixed typo for stringify

* documentation for native

* Review comments from Prebid

* Typo

* Typo

* Updated pub id for native

* Code Review