feature: Hidden field

[mihaimdm22]

Let me know what you think about this. Thought its needed only on edit.

[adrianthedev]

Just a few nitpicks, and one more test to make sure that the input works.

[codecov]

Codecov Report

Merging #71 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   95.96%   96.02%   +0.05%     
==========================================
  Files          85       87       +2     
  Lines        1861     1886      +25     
==========================================
+ Hits         1786     1811      +25     
  Misses         75       75              

Impacted Files	Coverage Δ
lib/avocado/app/fields/hidden_field.rb	100.00% <100.00%> (ø)
lib/avocado/app/resource_fields.rb	100.00% <100.00%> (ø)
spec/dummy/app/services/avocado/resources/user.rb	100.00% <100.00%> (ø)
spec/system/avocado/avocado_hidden_field_spec.rb	100.00% <100.00%> (ø)

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