SNOW-1256908 Handle int64 correctly in structured types

[sfc-gh-pfus]

Description

SNOW-1256908 When JSON is being deserialized, by default all numbers are deserialized into into float64 and for int64 precision is lost. This change deserializes to string.

Checklist

• [x] Created tests which fail without the change (if possible)
• [x] Extended the README / documentation, if necessary

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 82.28%. Comparing base (cec541e) to head (5323dfc).

Files	Patch %	Lines
structured_type.go	0.00%	7 Missing :warning:
converter.go	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1088 +/- ## ========================================== - Coverage 82.33% 82.28% -0.05% ========================================== Files 52 52 Lines 10195 10201 +6 ========================================== Hits 8394 8394 - Misses 1801 1807 +6 ```

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