relevantBidAdapter: add useSourceBidderCode option

[hugolm84]

Add opt-in useSourceBidderCode bidder param that passes the upstream bidder's seat through in bid responses. This allows alternate bidder codes to work when relevantdigital is used as a server-side adapter.

Documented here https://docs.prebid.org/dev-docs/bidders/relevantdigital.html

[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, 9bd8753

relevantdigital

Refer here for heat map coverage report

github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:46:	Builder				100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:57:	patchBidRequestExt		68.2%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:102:	patchBidImpExt			100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:106:	setTMax				100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:115:	createBidRequest		100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:139:	createJSONRequest		91.7%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:165:	getImpressionExt		100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:181:	buildEndpointURL		100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:190:	buildAdapterRequest		85.7%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:211:	MakeRequests			100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:228:	getImpressionsInfo		100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:240:	getHeaders			100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:260:	getMediaTypeForBidFromExt	100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:271:	getMediaTypeForBid		100.0%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:286:	isSupportedMediaType		83.3%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:300:	useSourceBidderCode		66.7%
github.com/prebid/prebid-server/v3/adapters/relevantdigital/relevantdigital.go:311:	MakeBids			83.3%
total:											(statements)			89.3%