rename system.SystemRule to system.Rule

[louyuting]

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[codecov-commenter]

Codecov Report

Merging #222 into master will decrease coverage by 0.06%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   42.63%   42.56%   -0.07%     
==========================================
  Files          84       84              
  Lines        4492     4485       -7     
==========================================
- Hits         1915     1909       -6     
+ Misses       2334     2333       -1     
  Partials      243      243              

Impacted Files	Coverage Δ
core/system/rule.go	90.00% <66.66%> (ø)
ext/datasource/helper.go	57.65% <80.00%> (ø)
core/system/rule_manager.go	94.23% <100.00%> (ø)
core/system/slot.go	56.60% <100.00%> (ø)
core/flow/rule_manager.go	51.58% <0.00%> (-2.21%)	⬇️
core/flow/rule.go	0.00% <0.00%> (ø)
core/flow/slot.go	0.00% <0.00%> (ø)

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[sczyh30]

LGTM