Add async APIs for shutting down the databases

[0xTim]

These changes are now available in 1.49.0

To enable removing calls to .wait() in the connection pools when in an async context

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 24.78%. Comparing base (cff1c3e) to head (22074e2).

Files	Patch %	Lines
Sources/FluentKit/Database/Databases.swift	0.00%	8 Missing ⚠️
Sources/FluentKit/Database/Database.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #614      +/-   ##
==========================================
- Coverage   24.80%   24.78%   -0.03%     
==========================================
  Files         149      149              
  Lines        8650     8660      +10     
==========================================
  Hits         2146     2146              
- Misses       6504     6514      +10     

Files	Coverage Δ
Sources/FluentKit/Database/Database.swift	73.68% <0.00%> (-8.67%)	⬇️
Sources/FluentKit/Database/Databases.swift	0.00% <0.00%> (ø)