Replace EL methods with concurrency

[ptoffy]

This updates FluentKit to use modern concurrency methods instead of EL-based code and switches the EL-based versions to use a/a under the hood.
The change is required by #645 but is big enough to warrant its own PR

[codecov]

Codecov Report

❌ Patch coverage is 50.15823% with 315 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.11%. Comparing base (0272fda) to head (d5fcc2f).

Files with missing lines	Patch %	Lines
...uentKit/Concurrency/QueryBuilder+Concurrency.swift	72.36%	42 Missing ⚠️
...luentKit/Concurrency/EnumBuilder+Concurrency.swift	0.00%	37 Missing ⚠️
...s/FluentKit/Concurrency/Siblings+Concurrency.swift	50.00%	34 Missing ⚠️
...s/FluentKit/Concurrency/AsyncModelMiddleware.swift	0.00%	21 Missing ⚠️
Sources/FluentKit/Properties/Siblings.swift	34.48%	19 Missing ⚠️
Sources/FluentKit/Properties/OptionalParent.swift	0.00%	17 Missing ⚠️
Sources/FluentKit/Properties/CompositeParent.swift	0.00%	16 Missing ⚠️
Sources/FluentKit/Properties/Children.swift	12.50%	14 Missing ⚠️
...rces/FluentKit/Concurrency/Model+Concurrency.swift	89.43%	13 Missing ⚠️
Sources/FluentKit/Properties/Parent.swift	13.33%	13 Missing ⚠️
... and 17 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
+ Coverage   24.35%   25.11%   +0.76%     
==========================================
  Files         148      148              
  Lines        8676     8838     +162     
==========================================
+ Hits         2113     2220     +107     
- Misses       6563     6618      +55     

Files with missing lines	Coverage Δ
Sources/FluentKit/Database/Database.swift	73.68% <ø> (ø)
Sources/FluentKit/Model/Model.swift	69.23% <100.00%> (+0.48%)	⬆️
Sources/FluentKit/Properties/Relation.swift	47.82% <100.00%> (-2.18%)	⬇️
...luentKit/Query/Builder/QueryBuilder+Paginate.swift	61.29% <100.00%> (-13.71%)	⬇️
Sources/FluentKit/Query/Builder/QueryBuilder.swift	74.85% <ø> (-4.10%)	⬇️
...ntKit/Concurrency/ModelResponder+Concurrency.swift	30.43% <83.33%> (+30.43%)	⬆️
...ces/FluentKit/Concurrency/Parent+Concurrency.swift	50.00% <50.00%> (+50.00%)	⬆️
...ntKit/Concurrency/OptionalParent+Concurrency.swift	0.00% <0.00%> (ø)
...s/FluentKit/Concurrency/Relation+Concurrency.swift	75.00% <71.42%> (+75.00%)	⬆️
...entKit/Concurrency/SchemaBuilder+Concurrency.swift	66.66% <66.66%> (ø)
... and 22 more

... and 3 files with indirect coverage changes

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