[feat] official json encode/decode as helper in dataconv

[hyorigo]

• for convt
• more comments
• draft tests
• fix tests

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.17% (target: -1.00%)	:white_check_mark: 94.74%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (196d60e6661a376154a98b656a12b46db9c6e006) | 4986 | 4643 | 93.12% | | | Head commit (bd553c8f0be9957fa60b8da4317f92a164e2604e) | 4937 (-49) | 4606 (-37) | 93.30% (**+0.17%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#112) | 19 | 18 | **94.74%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.00%. Comparing base (196d60e) to head (bd553c8).

Files	Patch %	Lines
dataconv/helper.go	90.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ========================================== + Coverage 90.72% 91.00% +0.27% ========================================== Files 41 41 Lines 4840 4789 -51 ========================================== - Hits 4391 4358 -33 + Misses 337 325 -12 + Partials 112 106 -6 ```

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