Migrate CI to use GitHub Actions.
### Motivation:
To migrate to GitHub actions and centralised infrastructure.
### Modifications:
Changes of note:
* Adopt swift-format using rules from SwiftNIO.
* Remove scripts and docker files which are no longer needed.
* Disabled warnings-as-errors on Swift 6.0 CI pipelines for now.
### Result:
Feature parity with old CI.
### Motivation
NIO may send `channelRead` events without a handlers requesting more data with a `context.read()` invocation. This happens if the remote has closed the connection and NIO wants to inform the handlers as soon as possible.
### Changes
- Don't `precondition` on `channelRead` events anymore.
- Close channel if we received an http end without a `context.read()` invocation
- `ConsumerControlState` was replaced with `ResponseStreamState` in `HTTPRequestStateMachine`
- `channelRead`s are buffered up to a `channelReadComplete`.
- on `channelReadComplete` all buffered body parts are forwarded to the consumer
Rick Newton-Rogers
Fabian Fett