Associate entry with `Payload`

[mylibrar]

Is your feature request related to a problem? Please describe.
Now that we can create multiple payloads, we should consider how to create new entry objects that are associated with a specific payload. We should also implement the Payload.get() method to retrieve entries from a specific Payload entry.

Describe the solution you'd like

• Design how to associate an entry with Payload. Below is one tentative solution:

  # Create a Sentence annotation on text payload at index 2
  
  # Solution 1:
  pack.set_text_payload_index(2)
  Sentence(pack, begin, end)
  
  # Solution 2:
  sentence = Sentence(pack, begin, end)
  sentence.text_payload_index = 2

• It should be compatible with the previous interfaces. When payload index is not specified, it should default to text payload at index 0.

• Design how to retrieve entries from Payload. Below is a tentative interface design of Payload.get:

  image_payload = pack.get_payload_at(1) 
  text_payload = pack.get_payload_at(2) 
  audio_payload = pack.get_payload_at(3) 
  
  image_payload.get(Box) 
  text_payload.get(Sentence) 
  audio_payload.get(Recording) 
  
  # This should return an empty iterator 
  image_payload.get(Recording) 

• Design the expected behavior of deleting a Payload entry

  • How to handle the entries associated with the deleted Payload?
  • Do we allow its payload index to be reused? For example, if we delete the payload at text_payload_list[3], can we reserve this position 3 for new text payload?

Describe alternatives you've considered
The design above is not final. We can discuss different solutions and their trade-offs.

Additional context
Include test cases to cover all common use cases.