GumGum: Override the default currency.

[MartinGumGum]

This PR resolves an issue where bid currency from the Ad Exchange was not being set in the Prebid Server's Go version, causing the currency to default to USD in all cases. The bid currency will now be dynamically set based on the response from the Ad Exchange, ensuring proper handling of different currencies in the bidding process.

[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, f481c0d

gumgum

Refer here for heat map coverage report

github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:23:	MakeRequests		87.1%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:84:	MakeBids		76.2%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:130:	preprocess		71.4%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:189:	getBiggerFormat		100.0%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:213:	getMediaTypeForImpID	100.0%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:223:	Builder			100.0%
total:									(statements)		82.2%

[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, 30c7c78

gumgum

Refer here for heat map coverage report

github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:23:	MakeRequests		87.1%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:84:	MakeBids		81.0%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:130:	preprocess		71.4%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:189:	getBiggerFormat		100.0%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:213:	getMediaTypeForImpID	100.0%
github.com/prebid/prebid-server/v2/adapters/gumgum/gumgum.go:223:	Builder			100.0%
total:									(statements)		83.2%