Make PSQLError public

[fabianfett]

Motivation

Users want to inspect what has gone wrong. For this they need access to the error type that we throw.

Changes

• Rewrite PSQLError to ensure it is a type that we can evolve in public
  • For this to work we shouldn't use associated types on the code enum
• Merge channel and connectionError code into one
• Rename PSQLDecodingError to PostgresMessageDecodingError to ensure a clear distinction to PostgresDecodingError
• PSQLError is now a CoW type
• Attach the query, if we have one to the PSQLError to make debugging easier

Result

• Better debuggable errors.

[gwynne]

Looks fine, but one nit: If you missed any other uses of #file (I dunno if you did, I didn't look), might as well change the rest to #fileID too (or #filePath if used within XCTest stuff) so there isn't any inconsistency.

[codecov-commenter]

Codecov Report

Merging #342 (1a057b0) into main (f21252b) will decrease coverage by 0.38%.
The diff coverage is 30.40%.

❗ Current head 1a057b0 differs from pull request most recent head 12bebe8. Consider uploading reports for the commit 12bebe8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
- Coverage   41.57%   41.19%   -0.38%     
==========================================
  Files         115      115              
  Lines        9434     9600     +166     
==========================================
+ Hits         3922     3955      +33     
- Misses       5512     5645     +133     

Impacted Files	Coverage Δ
Sources/PostgresNIO/Postgres+PSQLCompat.swift	0.00% <0.00%> (ø)
...es/PostgresNIO/Connection/PostgresConnection.swift	16.63% <3.84%> (-0.70%)	⬇️
...urces/PostgresNIO/New/PostgresChannelHandler.swift	61.14% <33.33%> (ø)
Sources/PostgresNIO/New/PSQLError.swift	43.36% <34.03%> (-37.65%)	⬇️
...nection State Machine/ConnectionStateMachine.swift	57.63% <80.00%> (+0.01%)	⬆️
...ion State Machine/AuthenticationStateMachine.swift	49.18% <100.00%> (ø)
...ostgresNIO/New/PostgresBackendMessageDecoder.swift	94.02% <100.00%> (ø)