Replace `pkg/client/generic` with dynamic client from `client-go`

[gary-lgy]

pkg/client/generic is legacy we inherited from KubeFed. It is an unnecessary layer of abstraction and has different usage patterns from client-go's dynamic client. We'd like to standardize all clients to those provided by client-go.

[gary-lgy]

All packages in pkg/client are generated except pkg/client/generic. After pkg/client/generic is removed, we should rename pkg/client to something like pkg/generated to better reflect its status.