feat:DCV-1933 changes to env_var and json flattening

[BAntonellini]

• Rename env_vars to env_var to match dbt-core naming
• Change json flattening flag type from Boolean to String, allowing 'yes', 'no' and 'ask'

[codecov-commenter]

Codecov Report

Attention: 231 lines in your changes are missing coverage. Please review.

Comparison is base (c9fcd60) 44.65% compared to head (21e7a6f) 45.95%.
Report is 93 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   44.65%   45.95%   +1.29%     
==========================================
  Files          34       35       +1     
  Lines        2770     2927     +157     
==========================================
+ Hits         1237     1345     +108     
- Misses       1533     1582      +49     

Flag	Coverage Δ
unittests	45.95% <46.15%> (+1.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
dbt_coves/__init__.py	100.00% <100.00%> (ø)
dbt_coves/config/config.py	89.93% <100.00%> (-0.07%)	⬇️
dbt_coves/tasks/dbt/main.py	41.09% <100.00%> (+1.65%)	⬆️
dbt_coves/tasks/extract/fivetran.py	38.70% <100.00%> (+2.04%)	⬆️
dbt_coves/tasks/generate/main.py	100.00% <ø> (ø)
dbt_coves/tasks/generate/metadata.py	23.60% <100.00%> (+0.96%)	⬆️
dbt_coves/tasks/generate/properties.py	34.12% <100.00%> (+1.06%)	⬆️
dbt_coves/tasks/generate/templates.py	44.68% <100.00%> (+2.45%)	⬆️
dbt_coves/tasks/setup/main.py	100.00% <100.00%> (ø)
dbt_coves/utils/jinja.py	100.00% <100.00%> (ø)
... and 16 more

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