Return different matrix types for online serving

Is your feature request related to a problem? Please describe.
We should allow Feature Views to return matrices/tensors natively. For example, torch.tensors.

At the moment, for some features we require the client to serialize the output into a matrix before running inference. Feast should support executing these transformations and serializing the data into matrices for both online and offline retrieval.

Describe the solution you'd like

features: torch.Tensor =  store.get_online_features()

Describe alternatives you've considered
Not supporting this is the alternative, which is the current state, which leaves users to write their own brittle logic to handle various complexities.

Additional context
@HaoXuAI @tokoko I know we discussed sklearn pipelines in the past and I thought I'd share my thoughts.