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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community) | Coverage Δ | | |---|---|---| | [...endGridKit/Models/AdvancedSuppressionManager.swift](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20?src=pr&el=tree&filepath=Sources%2FSendGridKit%2FModels%2FAdvancedSuppressionManager.swift&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community#diff-U291cmNlcy9TZW5kR3JpZEtpdC9Nb2RlbHMvQWR2YW5jZWRTdXBwcmVzc2lvbk1hbmFnZXIuc3dpZnQ=) | `100.00% <100.00%> (+100.00%)` | :arrow_up: | | [Sources/SendGridKit/Models/EmailAttachment.swift](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20?src=pr&el=tree&filepath=Sources%2FSendGridKit%2FModels%2FEmailAttachment.swift&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community#diff-U291cmNlcy9TZW5kR3JpZEtpdC9Nb2RlbHMvRW1haWxBdHRhY2htZW50LnN3aWZ0) | `100.00% <100.00%> (ø)` | | | [Sources/SendGridKit/Models/SendGridEmail.swift](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20?src=pr&el=tree&filepath=Sources%2FSendGridKit%2FModels%2FSendGridEmail.swift&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community#diff-U291cmNlcy9TZW5kR3JpZEtpdC9Nb2RlbHMvU2VuZEdyaWRFbWFpbC5zd2lmdA==) | `100.00% <100.00%> (ø)` | | | [Sources/SendGridKit/SendGridClient.swift](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20?src=pr&el=tree&filepath=Sources%2FSendGridKit%2FSendGridClient.swift&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community#diff-U291cmNlcy9TZW5kR3JpZEtpdC9TZW5kR3JpZENsaWVudC5zd2lmdA==) | `100.00% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/vapor-community/sendgrid-kit/pull/20/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor-community)