make syncer interface, add mocked syncer and watcher

[suchen-sci]

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[codecov-commenter]

Codecov Report

Merging #507 (e880365) into main (e2a96b8) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
+ Coverage   80.52%   80.56%   +0.03%     
==========================================
  Files          87       87              
  Lines       10123    10123              
==========================================
+ Hits         8152     8156       +4     
+ Misses       1516     1514       -2     
+ Partials      455      453       -2     

Impacted Files	Coverage Δ
pkg/cluster/cluster_interface.go	76.92% <ø> (ø)
pkg/cluster/syncer.go	77.56% <100.00%> (ø)
pkg/filter/headerlookup/headerlookup.go	88.81% <100.00%> (ø)
pkg/filter/validator/basicauth.go	78.89% <100.00%> (ø)
pkg/object/autocertmanager/storage.go	64.84% <100.00%> (ø)
pkg/object/mqttproxy/client.go	79.63% <0.00%> (-0.91%)	⬇️
pkg/cluster/cluster.go	52.89% <0.00%> (+1.02%)	⬆️

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