Make Postgres async query public

[fabianfett]

Follow up to #248. Last step in making the new Query API public.

[gwynne]

🎉🎉🎉!!!

[codecov-commenter]

Codecov Report

Merging #249 (ab77d02) into main (3d6666d) will increase coverage by 4.03%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
+ Coverage   39.74%   43.77%   +4.03%     
==========================================
  Files         115      115              
  Lines        9499     9506       +7     
==========================================
+ Hits         3775     4161     +386     
+ Misses       5724     5345     -379     

Flag	Coverage Δ
unittests	43.77% <93.75%> (+4.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...es/PostgresNIO/Connection/PostgresConnection.swift	15.42% <0.00%> (+0.72%)	⬆️
Sources/PostgresNIO/New/PostgresQuery.swift	86.66% <100.00%> (ø)
...ces/PostgresNIO/New/PostgresRow-multi-decode.swift	4.44% <0.00%> (+0.19%)	⬆️
Sources/PostgresNIO/Data/PostgresRow.swift	50.00% <0.00%> (+1.02%)	⬆️
...rces/PostgresNIO/New/Extensions/Logging+PSQL.swift	19.60% <0.00%> (+1.42%)	⬆️
...s/PostgresNIO/New/Data/Array+PostgresCodable.swift	93.25% <0.00%> (+2.24%)	⬆️
...ostgresNIO/Deprecated/PostgresMessageDecoder.swift	95.45% <0.00%> (+2.27%)	⬆️
...es/PostgresNIO/New/PSQLBackendMessageDecoder.swift	93.28% <0.00%> (+2.98%)	⬆️
...urces/PostgresNIO/New/Messages/ErrorResponse.swift	32.30% <0.00%> (+3.07%)	⬆️
... and 22 more