ENR Client Identifier

[jrhea]

Tracking the distribution of client types/versions is difficult; however, this could be made much easier if we simply added a client identifier field to the ENR. The benefits mainly fall under the category of network health monitoring and the downside is that this makes it easier for attackers to target nodes by client/version. I am opening this issue so that we can discuss the pros and cons.

My opinion is that this is identifier is probably only advantageous for an attacker targeting a zero-day vulnerability. If they are targeting a known vulnerability that is fixed in a newer version of the client, then we will also know that there are vulnerable clients on the network and we can campaign to have people update their clients. In other words, i don't think this gives an attacker much (or any) of an edge, but it does give us useful information that we can use to monitor network health.

cc: @ralexstokes, @dankrad, @djrtwo, @protolambda, @mkalinin, @AgeManning, @arnetheduck, @Nashatyrev