Enable use of idle connection pruning

[gwynne]

These changes are now available in 4.8.0

AsyncKit 1.21.0 added support for automatic pruning of idle database connections in the connection pool. This PR adds the necessary configuration parameters to the MySQL driver, enabling Fluent users to take advantage of this new functionality:

app.databases.use(.mysql(
    url: "mysql://user@pass:host/db",
    pruningInterval: .minutes(1),
    maxIdleTimeBeforePruning: .minutes(10)
), as: .mysql)

The pruningInterval controls how often the pool will check for idle connections; specifying nil disables idle pruning entirely. The maxIdleTimeBeforePruning controls how long a connection must go unused before being pruned.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.18%. Comparing base (c1422fd) to head (29febb9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
+ Coverage   97.61%   98.18%   +0.56%     
==========================================
  Files           7        7              
  Lines         252      331      +79     
==========================================
+ Hits          246      325      +79     
  Misses          6        6              

Files with missing lines	Coverage Δ
...s/FluentMySQLDriver/FluentMySQLConfiguration.swift	100.00% <100.00%> (ø)
...ources/FluentMySQLDriver/FluentMySQLDatabase.swift	97.97% <100.00%> (+0.17%)	⬆️

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