Add MongoDB dependencies to vendor

vendor/github.com/xdg-go/pbkdf2/pbkdf2.go

vendor/github.com/xdg-go/scram/.gitignore

vendor/github.com/xdg-go/scram/CHANGELOG.md

vendor/github.com/xdg-go/scram/LICENSE

vendor/github.com/xdg-go/scram/README.md

vendor/github.com/xdg-go/scram/client.go

vendor/github.com/xdg-go/scram/client_conv.go

vendor/github.com/xdg-go/scram/common.go

vendor/github.com/xdg-go/scram/doc.go

vendor/github.com/xdg-go/scram/parse.go

vendor/github.com/xdg-go/scram/scram.go

vendor/github.com/xdg-go/scram/server.go

vendor/github.com/xdg-go/scram/server_conv.go

vendor/github.com/xdg-go/stringprep/.gitignore

vendor/github.com/xdg-go/stringprep/CHANGELOG.md

vendor/github.com/xdg-go/stringprep/LICENSE

vendor/github.com/xdg-go/stringprep/README.md

vendor/github.com/xdg-go/stringprep/bidi.go

vendor/github.com/xdg-go/stringprep/doc.go

vendor/github.com/xdg-go/stringprep/error.go