Fix destreceiver api

[aykut-bozkurt]

• We need to put *mut ParquetWriterContext into PgParquetDestReciever. As it is opaque type to C users, it is safe to do and way safer than static context which causes headache when 2 concurrent dest receiver is running.
• PgParquetDestReceiver should not own and drop tupledesc that is received via its startup callback. That is unexpected for the caller.
• We should not filter our dropped columns of the given tupledesc at startup callback since it cause "tupledesc does not match tuple" errors.

[codecov]

Codecov Report

Attention: Patch coverage is 91.22807% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.64%. Comparing base (30fb0c6) to head (54d4787).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parquet_copy_hook/copy_to_dest_receiver.rs	90.24%	4 Missing ⚠️
src/parquet_copy_hook/hook.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   92.65%   92.64%   -0.01%     
==========================================
  Files          75       75              
  Lines        9813     9795      -18     
==========================================
- Hits         9092     9075      -17     
+ Misses        721      720       -1     

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