Allow peer to have additional protocols (e.g. ES indexation)

Allow a peer document to declare additional protocol (not BASIC_MERKLE_API). e.g. add a ElasticSearch indexation, for mobile application.

=> using a config entry ?

Use : a client app (e.g. mobile app) could found (using the signed peer document) that a node has optimized features enabled, and use it. (BMA need many HTTP request to get all user data)