feat: add Resolver#AddZone() method

[bryanchriswhite]

Changes

[Olshansk]

@ix64 @trois-six @foxcpp Any chance we can get a review on this or help by becoming repo maintainers?

[kd7lxl]

Would be nice if this canonicalized the name, since that's what the server tries to match during lookups.

Suggested change

	name = dns.CanonicalName(name)

[horkhe]

An alternative solution for the thread-safety issue has just been accepted from #21. Instead of introducing a mutex in Resolver it was Server that was made thread-safe and featuring AppendRR, DeleteRR and Reset functions for runtime zone manipulation.

At first I wanted to make Resolver thread-safe but without making the Zones field private that was impossible to achieve. So rather than breaking the package API I decided to follow the path of least resistance and update Server. It could be good enough for you too.