Add DNS resolver field

[nusenu]

• use-case: provide data in case of wide scale DNS problems
• field is only relevant for exit relays
• should contain the software (unbound, bind, ...)
• configuration?

[nusenu]

in d60f5b622cc4555a872f84e4d0d842a4823ebdc7 we added DNS related fields, even if they don't contain the used software.