Dgraph Alpha group stores can be replaced via unauthenticated external snapshot import

Dgraph is an open source distributed GraphQL database. Prior to version 25.3.5, Dgraph Alpha exposes the RPCs used for external snapshot import on the public gRPC port `:9080` without authentication or authorization. As a result, an unauthenticated network client can open `StreamExtSnapshot` and send Badger stream data to the target group’s store. In addition, the receiver calls `Prepare()` before processing the stream. This operation deletes and replaces the existing DB data. Version 25.3.5 patches the issue.