fix: Sink schema comparison before adding metadata columns

[pnadolny13]

Closes #1777

• moves test mock classes from the csv test to conftest so its reusable. Let me know if this should go somewhere else.
• adds test for target_base get_sink method to assert that when add_metadata_columns is used it doesnt affect the comparison. The tests fail in CI at first, then pass once I added a fix
• I chose to fix it by adding an original_schema attribute to the sink object. The alternative, and what I did in target-snowflake originally, is to add logic to remove those columns before doing the comparison but that sounded like more work.

@edgarrmondragon let me know what you think!

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📚 Documentation preview 📚: https://meltano-sdk--1778.org.readthedocs.build/en/1778/

[codecov]

Codecov Report

Merging #1778 (bfeb482) into main (88d7586) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1778      +/-   ##
==========================================
+ Coverage   85.93%   86.12%   +0.18%     
==========================================
  Files          59       59              
  Lines        4942     4944       +2     
  Branches      809      809              
==========================================
+ Hits         4247     4258      +11     
+ Misses        492      485       -7     
+ Partials      203      201       -2     

Impacted Files	Coverage Δ
singer_sdk/target_base.py	91.12% <ø> (+0.93%)	⬆️
singer_sdk/sinks/core.py	84.41% <100.00%> (+4.81%)	⬆️

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[edgarrmondragon]

This is great @pnadolny13 😁