Swift 6 update

[fpseverino]

• Update package to Swift 6.0
• Switch from XCTest to Swift Testing
• Add more test coverage
• Update to latest AsyncHTTPClient APIs
• Add support for EU regional subusers
• Improve DocC

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c7dda56) to head (a87da0d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main       #20       +/-   ##
============================================
+ Coverage   69.44%   100.00%   +30.55%     
============================================
  Files           9         9               
  Lines         108       104        -4     
============================================
+ Hits           75       104       +29     
+ Misses         33         0       -33     

Files with missing lines	Coverage Δ
...endGridKit/Models/AdvancedSuppressionManager.swift	100.00% <100.00%> (+100.00%)	⬆️
Sources/SendGridKit/Models/EmailAttachment.swift	100.00% <100.00%> (ø)
Sources/SendGridKit/Models/SendGridEmail.swift	100.00% <100.00%> (ø)
Sources/SendGridKit/SendGridClient.swift	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[MahdiBM]

Some discussions