Add an async query API (internal for now)

[fabianfett]

Motivation

We like async code

Changes

• Add an async query API to PostgresConnection (internal for now)
• Add first two async integration tests

Result

• Async is closer to land publicly

[codecov-commenter]

Codecov Report

Merging #233 (adb7107) into main (de78097) will increase coverage by 4.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   40.24%   44.26%   +4.01%     
==========================================
  Files         115      115              
  Lines        9320     9349      +29     
==========================================
+ Hits         3751     4138     +387     
+ Misses       5569     5211     -358     

Flag	Coverage Δ
unittests	44.26% <0.00%> (+4.01%)	⬆️

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

Impacted Files	Coverage Δ
...es/PostgresNIO/Connection/PostgresConnection.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/New/PSQLRow-multi-decode.swift	4.44% <0.00%> (+0.19%)	⬆️
Sources/PostgresNIO/Data/PostgresRow.swift	50.00% <0.00%> (+1.02%)	⬆️
...s/PostgresNIO/New/Data/Array+PostgresCodable.swift	94.68% <0.00%> (+1.06%)	⬆️
...rces/PostgresNIO/New/Extensions/Logging+PSQL.swift	19.60% <0.00%> (+1.42%)	⬆️
Sources/PostgresNIO/New/PSQLConnection.swift	30.00% <0.00%> (+2.10%)	⬆️
...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%)	⬆️
Sources/PostgresNIO/New/PostgresCell.swift	96.77% <0.00%> (+3.22%)	⬆️
... and 22 more

[gwynne]

Okay, now you're just outright teasing me. I demand perf benchmarks, including before/after you started working on the package altogether!