Stroeercore: Use bid.ext as-is from the response

[philipwatson]

Allows us to add to the bid ext without needing to update the adapter, which is especially convenient when adding fields for custom targeting. We intend to move dsa into ext in the near future, but we want to establish the PBS-Go version first.

Equivalent PBS-Java PR: prebid/prebid-server-java#4317

[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, b63546b

stroeerCore

Refer here for heat map coverage report

github.com/prebid/prebid-server/v3/adapters/stroeerCore/stroeercore.go:41:	resolveMediaType	100.0%
github.com/prebid/prebid-server/v3/adapters/stroeerCore/stroeercore.go:52:	MakeBids		88.2%
github.com/prebid/prebid-server/v3/adapters/stroeerCore/stroeercore.go:101:	getBidExt		100.0%
github.com/prebid/prebid-server/v3/adapters/stroeerCore/stroeercore.go:114:	MakeRequests		70.0%
github.com/prebid/prebid-server/v3/adapters/stroeerCore/stroeercore.go:154:	Builder			100.0%
total:										(statements)		84.3%