impl(bigquery): Json parsing changes for custom BigQuery library

[jsrinnn]

This PR modifies the SafeGetToNullable method based on the calling use case. The Json value field can hold any values and it is best parsed on the calling layer based on column schema and the row data that is provided to the caller.

The function is being modified to consistently return a string value. What that string value means is decided and parsed by by the calling layer as it has the most information to parse the value. It is not feasible to do this parsing at the Client LIbrary layer for our usecase.

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.91%. Comparing base (ae1e1e9) to head (e1e68a4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14918      +/-   ##
==========================================
- Coverage   92.91%   92.91%   -0.01%     
==========================================
  Files        2351     2351              
  Lines      209702   209741      +39     
==========================================
+ Hits       194851   194881      +30     
- Misses      14851    14860       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.