Leverage improvements in SQLKit, SQLiteNIO, and SQLiteKit

[gwynne]

These changes are now available in 4.7.0

Several improvements:

• Swift minimum version is now 5.8, minimum dependency versions updated
• Sendable and ExistentialAny correctness
• README and docs cleanup, updated CI
• Added support for custom JSON encoders and decoders
• Added working support for setting the query logging level
• Most operations are now routed through SQLiteKit rather than duplicating code, making behavior more consistent (and very slightly faster)
• The new extended SQLite errors are now supported correctly (DatabaseError.isConstraintFailure works again).
• The maxConnectionsPerEventLoop setting is now ignored, as increasing it beyond 1 increases thread contention with no additional parallelism.

[codecov]

Codecov Report

Attention: Patch coverage is 78.28947% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 82.54%. Comparing base (d76674f) to head (7107315).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   86.42%   82.54%   -3.88%     
==========================================
  Files           7        7              
  Lines         221      212       -9     
==========================================
- Hits          191      175      -16     
- Misses         30       37       +7     

Files	Coverage Δ
Sources/FluentSQLiteDriver/Exports.swift	100.00% <100.00%> (ø)
...s/FluentSQLiteDriver/SQLiteConverterDelegate.swift	100.00% <100.00%> (ø)
...rces/FluentSQLiteDriver/SQLiteError+Database.swift	95.23% <100.00%> (+11.90%)	⬆️
...ources/FluentSQLiteDriver/FluentSQLiteDriver.swift	72.72% <72.72%> (+6.06%)	⬆️
...ources/FluentSQLiteDriver/SQLiteRow+Database.swift	63.63% <57.89%> (-29.70%)	⬇️
...FluentSQLiteDriver/FluentSQLiteConfiguration.swift	77.77% <65.51%> (-22.23%)	⬇️
...rces/FluentSQLiteDriver/FluentSQLiteDatabase.swift	86.36% <85.54%> (+2.01%)	⬆️