feat: add custom field clean method

[mehanizm]

This PR has three commits that need the review.

feat: add RemoveIDCustomField method

In this commit I've added the method to delete (only clear) custom field of the card by its ID (see "Clearing CustomFieldItems" here).

But there was a problem. Because of the definition of the Value field in the CustomFieldItem result from the API was not parsed correctly. I'd tried to understand why we need here (in general purpose library) so complex unmarshaling with some kind of the business logic and stuck with that. There are no tests to explain this.

impr: change custom fields value struct

To develop this idea I've simplified CustomFieldItem struct and all corresponding methods. No tests was broken after that, only type actualization.

impr: refactor CustomFields method

As a result I've refactor CustomFields card method to use more idiomatic go code and new simplified CustomFieldItem struct.

Conclusion

I think, that it is more correct style for the general library and type casting from the CustomFieldItemValue should be done in the business logic on the application level. The casting logic that was here earlier is hard to understand and did not have general application.

[codecov-io]

Codecov Report

Merging #62 into master will increase coverage by 4.62%. The diff coverage is 82.50%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   73.25%   77.88%   +4.62%     
==========================================
  Files          21       21              
  Lines         890      850      -40     
==========================================
+ Hits          652      662      +10     
+ Misses        206      156      -50     
  Partials       32       32              

Impacted Files	Coverage Δ
custom-fields.go	100.00% <ø> (+83.87%)	:arrow_up:
card.go	59.16% <82.50%> (+1.64%)	:arrow_up:

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