pkg/flags: clean up, add "SelectiveStringsValue"

[gyuho]

To support #9216.

[codecov-io]

Codecov Report

Merging #9457 into master will increase coverage by 0.38%.
The diff coverage is 85.5%.

@@            Coverage Diff             @@
##           master    #9457      +/-   ##
==========================================
+ Coverage   72.53%   72.91%   +0.38%     
==========================================
  Files         363      364       +1     
  Lines       30848    30874      +26     
==========================================
+ Hits        22376    22513     +137     
+ Misses       6852     6755      -97     
+ Partials     1620     1606      -14

Impacted Files	Coverage Δ
pkg/flags/flag.go	88.67% <ø> (+3.38%)	⬆️
etcdmain/config.go	81.92% <100%> (ø)	⬆️
pkg/flags/ignored.go	100% <100%> (ø)
pkg/flags/urls.go	90% <100%> (+1.11%)	⬆️
pkg/flags/selective_string.go	80% <80%> (ø)
pkg/flags/strings.go	84.61% <84.61%> (-15.39%)	⬇️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
proxy/grpcproxy/lease.go	80.54% <0%> (-4.08%)	⬇️
raft/raft.go	90.99% <0%> (-0.7%)	⬇️
... and 26 more

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