Add a String() to Set and Array so it looks better on UI and symetrical with coma input

[ldemailly]

Also added tests and example

Also adding a bunch of Set functions (NewSet, Add, Has, Remove, Clone) into a new fortio.org/sets package

[codecov-commenter]

Codecov Report

Base: 83.83% // Head: 83.96% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (955f8fb) compared to base (867970e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   83.83%   83.96%   +0.12%     
==========================================
  Files          14       14              
  Lines         532      530       -2     
==========================================
- Hits          446      445       -1     
+ Misses         72       71       -1     
  Partials       14       14              

Impacted Files	Coverage Δ
endpoint/endpoint.go	58.51% <ø> (ø)
dyngeneric.go	100.00% <100.00%> (+1.45%)	⬆️
dynstringset.go	56.25% <100.00%> (ø)
configmap/updater.go	77.16% <0.00%> (-0.79%)	⬇️

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