refactor: make violations field of summary lint report record as integer

[dkrutskikh]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[codecov]

Codecov Report

Merging #544 (e501e85) into master (6bb140d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head e501e85 differs from pull request most recent head 3ffedea. Consider uploading reports for the commit 3ffedea to get more accurate results

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
+ Coverage   84.93%   84.94%   +0.01%     
==========================================
  Files         218      218              
  Lines        4460     4458       -2     
==========================================
- Hits         3788     3787       -1     
+ Misses        672      671       -1     

Impacted Files	Coverage Δ
...ics/metrics_list/maintainability_index_metric.dart	100.00% <ø> (ø)
...rics/metrics_list/number_of_parameters_metric.dart	100.00% <ø> (ø)
...r/metrics/metrics_list/weight_of_class_metric.dart	100.00% <ø> (ø)
...analyzers/lint_analyzer/metrics/models/metric.dart	100.00% <ø> (ø)
...nt_analyzer/models/summary_lint_report_record.dart	100.00% <ø> (ø)
...orters/reporters_list/json/lint_json_reporter.dart	100.00% <100.00%> (+1.38%)	⬆️

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