Protocol changes: Refactor Liveliness implementation

[fuzzypixelz]

A Zenoh application that declares a Liveliness token is monitored internally as if it were a subscriber. However, today the Zenoh protocol defines messages for declaring/undeclaring a "token".

The scope of this issue is to refactor the Liveliness implementation to use DeclareToken/UndeclareToken messages instead of DeclareSubscriber/UndeclareSubscriber messages.

The main motivator for this is the current interaction between the Liveliness feature and the aggregation configuration option, where a user could technically aggregate subscribers under @/liveliness/** which means that a Liveliness subscriber would not be able to distinguish between different monitored Zenoh applications.