feat: add csv() modifier

[anand2312]

Notes:

• Adding a .csv() to the chain will eventually return a SingleAPIResposne[str], whose data field will be the data returned as a CSV string.
• Calling csv() has a similar effect as calling single() - it has to be the last call in the chain (only .execute() can follow it)

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (5ae0f99) 91.36% compared to head (5906eaf) 91.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   91.36%   91.62%   +0.25%     
==========================================
  Files          24       24              
  Lines        1216     1253      +37     
==========================================
+ Hits         1111     1148      +37     
  Misses        105      105              

Files	Coverage Δ
postgrest/__init__.py	100.00% <ø> (ø)
postgrest/_async/request_builder.py	79.27% <100.00%> (+0.57%)	⬆️
postgrest/_sync/request_builder.py	85.58% <100.00%> (+0.40%)	⬆️
postgrest/base_request_builder.py	78.22% <100.00%> (+0.29%)	⬆️
tests/_async/test_request_builder.py	100.00% <100.00%> (ø)
tests/_sync/test_request_builder.py	100.00% <100.00%> (ø)

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[J0]

Need to add docs as well - I'll look into it when a slot frees up. We should probably record a video on how to update the docs at some point as well