Add Server type to entry

[jdknives]

Feature description

Dmsg allows outside entities to register their own dmsg.Server in our discovery. For certain clients it may be important to chose a specific type of servers (i.e. one maintained by Skycoin). It was requested by @SkycoinSynth that we add info in a given server entry about the type of server. The basic values for now would be community and official. For now it would be sufficient to accept self-reported information from servers. No verification is needed.

[0pcom]

This describes a whitelist of dmsg servers. To specify to the dmsg-discovery when it's started which dmsg servers that might connect to it are official