Add rewrite support to custom DNS

[ThinkChaos]

Main commit message:

This commit extracts rewriting logic from ConditionalUpstreamResolver
into the new RewriterResolver, and uses that to enable rewriting for
the CustomDNSResolver.
RewriterResolver wraps a resolver and applies the rewrite to the
request that is forwarded to the inner resolver.

It also introduces a new optional interface: NamedResolver.
This allows a Resolver to choose what its user friendly name is,
instead of always being its type name.

I use this feature to have an alternative way of spelling my domain which is a bit long.
Comes in handy when manually poking around.

Also did some smaller changes to tests such as unifying MockResolver and resolverMock.

[codecov]

Codecov Report

Merging #449 (dc9ccd5) into development (6d33ec7) will increase coverage by 1.30%.
The diff coverage is 97.59%.

@@               Coverage Diff               @@
##           development     #449      +/-   ##
===============================================
+ Coverage        94.26%   95.57%   +1.30%     
===============================================
  Files               33       35       +2     
  Lines             2722     2780      +58     
===============================================
+ Hits              2566     2657      +91     
+ Misses             118       85      -33     
  Partials            38       38              

Impacted Files	Coverage Δ
resolver/rewriter_resolver.go	96.66% <96.66%> (ø)
config/config.go	88.51% <100.00%> (ø)
resolver/blocking_resolver.go	98.41% <100.00%> (ø)
resolver/conditional_upstream_resolver.go	100.00% <100.00%> (ø)
resolver/custom_dns_resolver.go	100.00% <100.00%> (ø)
resolver/noop_resolver.go	100.00% <100.00%> (ø)
resolver/parallel_best_resolver.go	100.00% <100.00%> (ø)
resolver/resolver.go	100.00% <100.00%> (ø)
server/server.go	89.22% <100.00%> (ø)
redis/redis.go	90.78% <0.00%> (+23.02%)	⬆️

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[ThinkChaos]

Force pushed to fix Configuration() output.
The failed test looks random and unrelated: I haven't seen this in any previous CI or local run.

[0xERR0R]

Thanks for your work. It looks good. 👍