Fix build break when missing netty-dns-resolver

[yueki1993]

Fixes: #1546

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  https://github.com/yueki1993/lettuce-test-PR1517

[yueki1993]

Here causes NoClassDefError because there are upcasts DnsAddressResolverGroup -> AddressResolverGroup) and DefaultDnsCnameCache -> DnsCnameCache.
I moved these to internal static class.

[codecov]

Codecov Report

Merging #1547 (8951a23) into main (34476f5) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1547      +/-   ##
============================================
- Coverage     78.68%   78.65%   -0.03%     
+ Complexity     6340     6336       -4     
============================================
  Files           476      476              
  Lines         21318    21319       +1     
  Branches       2333     2333              
============================================
- Hits          16774    16769       -5     
- Misses         3472     3475       +3     
- Partials       1072     1075       +3     

Impacted Files	Coverage Δ	Complexity Δ
...ce/core/resource/AddressResolverGroupProvider.java	73.33% <100.00%> (+1.90%)	2.00 <0.00> (-1.00)	⬆️
.../java/io/lettuce/core/protocol/CommandHandler.java	73.60% <0.00%> (-2.04%)	110.00% <0.00%> (-4.00%)
...a/io/lettuce/core/protocol/ConnectionWatchdog.java	77.77% <0.00%> (-1.97%)	38.00% <0.00%> (-1.00%)
...ce/core/masterreplica/SentinelTopologyRefresh.java	84.96% <0.00%> (-0.66%)	35.00% <0.00%> (ø%)
...java/io/lettuce/core/protocol/DefaultEndpoint.java	70.06% <0.00%> (+0.46%)	99.00% <0.00%> (ø%)
.../io/lettuce/core/dynamic/ReactiveTypeAdapters.java	88.26% <0.00%> (+0.86%)	1.00% <0.00%> (ø%)
.../io/lettuce/core/protocol/CommandExpiryWriter.java	76.92% <0.00%> (+3.84%)	17.00% <0.00%> (+2.00%)

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