Implement workflow_step

[Simply007]

Motivation

Fixes #97

Checklist

• [ ] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [ ] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[codecov[bot]]

Codecov Report

Merging #102 (3cf913f) into master (4691c54) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #102   +/-   ##
=========================================
  Coverage     98.13%   98.13%           
- Complexity      194      195    +1     
=========================================
  Files            22       22           
  Lines           589      591    +2     
=========================================
+ Hits            578      580    +2     
  Misses           11       11           

Impacted Files	Coverage Δ
src/Kentico/Kontent/Delivery/DefaultMapper.php	98.46% <100.00%> (+0.02%)	:arrow_up:
...ontent/Delivery/Models/Items/ContentItemSystem.php	100.00% <100.00%> (ø)

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