refactor(rust): enable requests to be messages

[etorreborre]

This PR adjusts the serialization / deserialization of Ockam messages.

Motivation

The initial motivation was to be able to define Request<T> as a type of Message for workers processing messages.
Some additional objectives are to:

• Decouple the general Encodable / Decodable interfaces from the Serializable / Deserializable implementations provided by the serde_bare library
• Highlight the fact that Requests are currently doubly-encoded, once as a Vec<u8> with minicbor and then serialized with serde_bare. Fixing this issue will be a one-line (or 2) change after this PR but it is a breaking change, so I left this for later.

Changes

The main changes are:

1. The removal of default instances for Encodable and Decodable.
2. The addition of an Encodable and Decodable for a Request<T> provided that T: Message.
   • As a result, all the messages being sent via requests now have their own Message instance (that explains the number of touched files)
3. The stricter typing of ask/tell/request/... methods for both requests and responses.
4. A two-steps decoding process of Requests in workers like the NodeManager to:
   • First decode the request as Request<Vec<u8>> (now by the virtue of Request<T> being a Message) in order to read the RequestHeader.
   • And then decode the request payload using a Decodable<T> instance we know the expected type based on the header. Note that this is done without having to know that the request body was encoded using minicbor.