Adnuntius Bid Adapter: Refactoring code

[mikael-lundin]

Moving code out to their own files so that it's a bit easier to read.

[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, cf1a565

adnuntius

Refer here for heat map coverage report

github.com/prebid/prebid-server/v2/adapters/adnuntius/adnuntius.go:22:			Builder			100.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/adnuntius.go:32:			MakeRequests		100.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/adnuntius.go:36:			MakeBids		81.8%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:16:		makeEndpointUrl		92.9%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:65:		getImpSizes		90.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:85:		setHeaders		100.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:101:		getSiteExtAsKv		80.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:111:		getGDPR			92.9%
github.com/prebid/prebid-server/v2/adapters/adnuntius/request_functions.go:139:		generateRequests	92.0%
github.com/prebid/prebid-server/v2/adapters/adnuntius/response_functions.go:16:		generateAdResponse	82.1%
github.com/prebid/prebid-server/v2/adapters/adnuntius/response_functions.go:88:		generateReturnExt	86.7%
github.com/prebid/prebid-server/v2/adapters/adnuntius/response_functions.go:122:	generateBidResponse	88.0%
total:											(statements)		89.4%