Support retrieving multiple resource types in a single command

[snorwin]

Currently, gwctl get does not support requesting multiple resource types in a single command. Attempting to do so results in an error, even though this workflow is commonly supported by tools like kubectl.

Example:

$ gwctl get gateway,httproute            
cannot specify more than one type, received types: [gateway httproute]%   

This limitation makes it harder to quickly inspect related resources together, especially when debugging or validating Gateway API configurations.

[chaijunkin]

/assign