EXCH-10878 Native support for OpenX bidder

[sergseven]

having the PR here to have the checks running

[sergseven]

@KarolSzawlisOpenX

places we can think about additional tests:

• int. tests https://github.com/openx/prebid-server-java/tree/master/src/test/resources/org/prebid/server/it/openrtb2/openx
• functional tests https://github.com/openx/prebid-server-java/tree/master/src/test/groovy/org/prebid/server/functional/tests/bidder/openx
• unit tests https://github.com/openx/prebid-server-java/tree/master/src/test/java/org/prebid/server/bidder/openx (already added some test cases)

[sergseven]

note: we are considering removing this test case since it seems to be duplicated with the unit tests and it looks like this test level is not dedicated to test the features, but integration only

ref: https://github.com/prebid/prebid-server-java/blob/master/docs/developers/code-style.md#bidder-smoke-tests

[bukrain]

LGTM

[gmiedlar-ox]

According to our recent conversation, maybe it would be worth to add a test case verifying what will be returned from makeHttpRequest when there is multi-ad-placement imp in bid request?