Refactor ChatService into runtime

[jeandudey]

• ChatService is now Runtime as it's only an interface to the Chat logic, the runtime is needed by the JNI and probably could be used for other languages without any change e.g.: Node.JS.
• Runtime has now it's own module, along with Identity, some libp2p types are re-exported for ergonomics and consistency.
• Added documentation and examples (which are tested by the CI) for Identity and Runtime.

The next step after this is to decouple Gossipsub behaviour from the Runtime and create a custom behaviour, this will allow us to extend it with mDNS, Kademlia, our custom peer discovery service, etc. Separating that from the Runtime will make locha-p2pd easier to program, and will lower the complexity for #8 .

[jeandudey]

I'll merge as this also fixes a bug on master. Any comments can be added here and I will address them on another PR :wink: