perf: Ensure raw_schema in stream mapper is immutable

[pnadolny13]

Closes #1961

When trying to figure out how to fix #1961 I realized it was caused by the raw_schema in the mapper being a shared object with the transformed_schema which is passed to get_sink then add_sink, ultimately to the sink itself where SDC columns are added. The rest of the codebase expects transformed_schema to be mutable but raw_schema to be immutable (or at least it reads that way to me). They both share the same reference to the original schema dict so assumptions break. The main case I ran into was in #1961 where target-postgres was constantly creating new sink instances for me when the schema wasnt actually changing.

[codecov]

Codecov Report

Merging #1962 (9197015) into main (97d97de) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1962   +/-   ##
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  Coverage   87.08%   87.08%           
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  Files          59       59           
  Lines        5112     5112           
  Branches      827      827           
=======================================
  Hits         4452     4452           
  Misses        465      465           
  Partials      195      195           

Files Changed	Coverage Δ
singer_sdk/mapper.py	80.67% <100.00%> (ø)

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[edgarrmondragon]

This reduced running time from 15s to 3s in my very simple load test 😮

[edgarrmondragon]

Included in https://pypi.org/project/singer-sdk/0.32.0b2/