Refactor: move pkg auth in internal

internal/auth/auth.go

internal/auth/auth_test.go

internal/auth/dbusers.go

internal/auth/inmemory.go

internal/auth/oauth2.go

internal/models/bankAccounts.go

internal/models/bankers.go

internal/models/clients.go

internal/models/models.go

internal/models/operations.go

internal/rootmux/admin_test.go

internal/rootmux/banker_test.go

internal/rootmux/client_test.go

internal/rootmux/rootmux.go

internal/rootmux/rootmux_test.go