remove serialization of zenoh types from zenoh-ext

[milyin]

Describe the release item

The serialization of Timestamp and Encoding was added to zenoh-ext mostly for plugins. But at least for encoding plugins needs more effective way to store it: plugins can use internal encoding representation while universal serialization should not be based on it. Therefore serialization in backends will be made inside. Therefore we can remove these types from zenoh-ext.

• Encoding and timestamp serialization remove from zenoh-ext #1520
  • encoding timestamp custom serialization zenoh-backend-filesystem#243
  • use own encoding and timestamp serializatiion zenoh-backend-rocksdb#231