Add support for proxying in `WebsocketClient`

[rnro]

Added support for TLS and plain text proxying of Websocket traffic.

• In the TLS case a CONNECT header is first sent establishing the proxied traffic.

• In the plain text case the modified URI in the initial upgrade request header indicates to the proxy server that the traffic is to be proxied.

• Use NIOWebSocketFrameAggregator to handle aggregating frame fragments. This brings with it more protections e.g. against memory exhaustion.

• Accompanying config has been added to support this change.

This change also includes some performance gains by reducing the allocation and copies necessary to send ByteBuffer and ByteBufferView through WebSocket.send.

• Sending ByteBuffer or ByteBufferView doesn’t require any allocation or copy of the data. Sending a String now correctly pre allocates the ByteBuffer if multibyte characters are present in the String.

• Remove custom random websocket mask generation which would only generate bytes between UInt8.min..<UInt8.max, therefore excluding UInt8.max i.e. 255.

[rnro]

Thanks for the review @0xTim. I've pushed the changes you requested and also some performance improvements in a separate commit.

[codecov-commenter]

Codecov Report

Merging #130 (777ef32) into main (e0faccd) will decrease coverage by 3.62%. The diff coverage is 78.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #130 +/- ## ========================================== - Coverage 71.15% 67.54% -3.62% ========================================== Files 5 6 +1 Lines 475 758 +283 ========================================== + Hits 338 512 +174 - Misses 137 246 +109 ``` | [Impacted Files](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [...bSocketKit/Concurrency/WebSocket+Concurrency.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvQ29uY3VycmVuY3kvV2ViU29ja2V0K0NvbmN1cnJlbmN5LnN3aWZ0) | `29.78% <ø> (ø)` | | | [Sources/WebSocketKit/WebSocket+Connect.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0K0Nvbm5lY3Quc3dpZnQ=) | `50.00% <36.84%> (-47.50%)` | :arrow_down: | | [Sources/WebSocketKit/WebSocket.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0LnN3aWZ0) | `72.87% <77.41%> (+1.09%)` | :arrow_up: | | [...urces/WebSocketKit/HTTPUpgradeRequestHandler.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvSFRUUFVwZ3JhZGVSZXF1ZXN0SGFuZGxlci5zd2lmdA==) | `72.05% <77.77%> (ø)` | | | [Sources/WebSocketKit/WebSocketHandler.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0SGFuZGxlci5zd2lmdA==) | `61.97% <85.18%> (+6.65%)` | :arrow_up: | | [Sources/WebSocketKit/WebSocketClient.swift](https://codecov.io/gh/vapor/websocket-kit/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9XZWJTb2NrZXRLaXQvV2ViU29ja2V0Q2xpZW50LnN3aWZ0) | `86.00% <90.55%> (+18.03%)` | :arrow_up: |

[rnro]

Sorry about the delay. The missing DocC comments should now be there.

[VaporBot]

These changes are now available in 2.8.0

[0xTim]

FYI nightly error is being tracked at https://github.com/apple/swift/issues/65064