feat: update descriptor result output

[ryanleecode]

Updates descriptor result output to the shape of

[
  typeof constantDescriptors,
  typeof storageDescriptors,
  typeof eventDescriptors,
  typeof errorDescriptors,
  typeof callDescriptors
]

[josepot]

We should update the @polkadot-api/client accordingly. Let me handle that one. Thanks @ryanleecode !