Add toString for api models

[XiongKezhi]

I'm not sure if this is the right (or easiest) way to write the toString for our api models since we have so many nullable fields.

[codecov]

Codecov Report

Merging #8 into master will increase coverage by 0.42%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master       #8      +/-   ##
============================================
+ Coverage     82.12%   82.54%   +0.42%     
- Complexity       44       49       +5     
============================================
  Files            10       10              
  Lines           207      212       +5     
  Branches          7        7              
============================================
+ Hits            170      175       +5     
  Misses           37       37              

Impacted Files	Coverage Δ	Complexity Δ
...va/io/jenkins/plugins/checks/api/ChecksAction.java	100.00% <100.00%> (ø)	5.00 <1.00> (+1.00)
...o/jenkins/plugins/checks/api/ChecksAnnotation.java	100.00% <100.00%> (ø)	12.00 <1.00> (+1.00)
...a/io/jenkins/plugins/checks/api/ChecksDetails.java	100.00% <100.00%> (ø)	10.00 <1.00> (+1.00)
...ava/io/jenkins/plugins/checks/api/ChecksImage.java	100.00% <100.00%> (ø)	5.00 <1.00> (+1.00)
...va/io/jenkins/plugins/checks/api/ChecksOutput.java	100.00% <100.00%> (ø)	8.00 <1.00> (+1.00)

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