Re-export DataverseApiAuthMechanism from public core surface

Adds DataverseApiAuthMechanism to the existing core/index.ts re-export
alongside ApiConfig so consumers don't have to deep-import it from
`@iqss/dataverse-client-javascript/dist/core/infra/repositories/ApiConfig`.

This is the SDK side of a small two-line change agreed with the
dataverse-frontend reusable-components track: once a prerelease ships
this export, the standalone uploader can import the enum from the
package's public surface. Until then, consumers can keep the deep
import.

Non-breaking additive change.

Author: ErykKul

src/core/index.ts

@@ -1,5 +1,5 @@
 export { ReadError } from './domain/repositories/ReadError'
 export { WriteError } from './domain/repositories/WriteError'
-export { ApiConfig } from './infra/repositories/ApiConfig'
+export { ApiConfig, DataverseApiAuthMechanism } from './infra/repositories/ApiConfig'
 export { DvObjectOwnerNode, DvObjectType } from './domain/models/DvObjectOwnerNode'
 export { PublicationStatus } from './domain/models/PublicationStatus'