remove predefined serialization of internal types

[milyin]

Describe the release item

New serialization API added in #1474 raised concerns about serialization of internal types, especiallly Encoding and Timestamp

This functionality was added mostly to support serialization of these types in pluings. But as @JEnoch noticed, the provided universal encoding serialization doesn't work well for storages - they need the most efficient way for storing the encoding, even using zenoh's internal representation. On the other hand we don't want to expose serialization of zenoh types which uses internal representation as it may cause future incompatibility

The proposal is:

• remove implementaion of Serialize for Encoding and Timestamp from zenoh-ext
• serialiaze Encoding in storage backends using internal feature, with full access to zenoh internal data
• make sure that user also can still easily serialize/seserialize these types as a tuple

[milyin]

@wyfo , @kydos your opinion?
This approach is sligtly disappointing for users: the storages will use more effective serialization then whcih is availablle through standard API. But this adantage comes with a price: the serialization used in storages is zenoh implementation dependent.