Add Sendable conformances to WebsocketKit

[0xTim]

Bump minimum Swift version to 5.6 and add Sendable conformances where appropriate

[0xTim]

@lukasa if you want to give this a once over to make sure I'm not doing anything stupid that would be great!

[codecov-commenter]

Codecov Report

Merging #131 (1bedf08) into main (7b6c5bc) will decrease coverage by 1.86%. The diff coverage is 93.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #131 +/- ## ========================================== - Coverage 85.78% 83.93% -1.86% ========================================== Files 6 6 Lines 626 641 +15 ========================================== + Hits 537 538 +1 - Misses 89 103 +14 ``` | [Impacted Files](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [Sources/WebSocketKit/WebSocket+Connect.swift](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0K0Nvbm5lY3Quc3dpZnQ=) | `98.59% <ø> (ø)` | | | [Sources/WebSocketKit/WebSocketHandler.swift](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0SGFuZGxlci5zd2lmdA==) | `62.50% <ø> (ø)` | | | [...bSocketKit/Concurrency/WebSocket+Concurrency.swift](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvQ29uY3VycmVuY3kvV2ViU29ja2V0K0NvbmN1cnJlbmN5LnN3aWZ0) | `54.16% <75.00%> (-19.37%)` | :arrow_down: | | [Sources/WebSocketKit/WebSocket.swift](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0LnN3aWZ0) | `88.73% <97.05%> (+0.98%)` | :arrow_up: | | [Sources/WebSocketKit/WebSocketClient.swift](https://app.codecov.io/gh/vapor/websocket-kit/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0Q2xpZW50LnN3aWZ0) | `95.34% <100.00%> (+0.05%)` | :arrow_up: |

[0xTim]

Ok @gwynne I think we're finally good and I've fixed the race conditions in the tests

[VaporBot]

These changes are now available in 2.10.0