Asyncification of Fluent provider

[gwynne]

These changes are now available in 4.9.0

Updates the Fluent provider package to be (mostly) Sendable-correct and switches MigrateCommand to being async to avoid dangerous misuse of EventLoopFuture.wait().

Bumps minimum Swift version to 5.7 (matching Vapor).

[0xTim]

Need to fix a Sendable annotation but otherwise LGTM

[codecov-commenter]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (8665428) 60.10% compared to head (a075e7f) 60.10%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #772   +/-   ##
=======================================
  Coverage   60.10%   60.10%           
=======================================
  Files          15       15           
  Lines         386      386           
=======================================
  Hits          232      232           
  Misses        154      154           

Files	Coverage Δ
.../ModelCredentialsAuthenticatable+Concurrency.swift	85.71% <100.00%> (ø)
Sources/Fluent/Fluent+History.swift	88.88% <ø> (ø)
Sources/Fluent/Fluent+Pagination.swift	100.00% <ø> (ø)
...urces/Fluent/ModelCredentialsAuthenticatable.swift	75.00% <100.00%> (ø)
Sources/Fluent/PageLimit.swift	66.66% <ø> (ø)
...rces/Fluent/Concurrency/Sessions+Concurrency.swift	0.00% <0.00%> (ø)
Sources/Fluent/ModelAuthenticatable.swift	0.00% <0.00%> (ø)
Sources/Fluent/ModelTokenAuthenticatable.swift	0.00% <0.00%> (ø)
Sources/Fluent/Fluent+Sessions.swift	77.41% <60.00%> (ø)
Sources/Fluent/FluentProvider.swift	79.31% <81.81%> (ø)
... and 2 more

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.