Adnuntius: Add multi-format and native support

[mikael-lundin]

This change is meant to add support for Native format and enable us to add video in the future. We've als made it so that types and helper functions are in their own files.

[github-actions]

Code coverage summary

Note:

• Prebid team doesn't anticipate tests covering code paths that might result in marshal and unmarshal errors
• Coverage summary encompasses all commits leading up to the latest one, c9e7a43

adnuntius

Refer here for heat map coverage report

github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:26:		Builder				100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:36:		generateRequests		91.1%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:161:		MakeRequests			100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:165:		generateBidResponse		92.9%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:251:		MakeBids			81.8%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:278:		generateAdResponse		78.3%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:15:	setHeaders			100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:31:	makeEndpointUrl			92.9%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:80:	getImpSizes			100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:101:	getSiteExtAsKv			80.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:111:	getGDPR				92.9%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:136:	generateReturnExt		86.7%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:171:	generateAdUnit			100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius_utils.go:190:	convertMarkupTypeToBidType	50.0%
total:										(statements)			89.5%