AppNexus Bid Adapter: Add support for custom Native fields

[JaroVDH]

Type of change

• Updated bidder adapter

Description of change

Includes AppNexus/Xandr's custom Native fields in the bid response's ext field.

Other information

This PR will also be forwarded to the Xandr teams for their approval.

[JaroVDH]

Small note/question: I used the spread syntax (...) in the tests.
Prebid.js supports IE 11, which doesn't support that.
I don't think it's an issue as it's part of the tests, and those run on node 12+.
If it is an issue, I'll quickly change it.

[JaroVDH]

Looks like the e2etest failed.
Locally, the same 6 tests failed before and after my changes, so I didn't expect a failure there.

[jsnellbaker]

Thank you for putting these changes together. LGTM