`tjson`: Implement `timestamp`

[AlekSi]

This task is a part of #683 epic. This task should be taken after #899 is done.

🎯 The goal of the task is to implement timestamp type for tjson.

Substasks:

• [ ] Support timestamp storing it as an object (document) with $t key (see tjson/binary.go and fjson/timestamp.go).

To Do:

• [ ] Add unit tests
  • [ ] Dedicated tests for datetime and timestamp (like in the fjson package)
  • [ ] Add datetime and timestamp fields to one of the test cases in the document_test.go file.
• [ ] Port integration tests