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.
- The connection creation logic has been refactored into a number of smaller methods that can be combined
- Connection creation now has a logical home. It is moved from `Utils.swift` into a `ConnectionFactory`
- There are explicit `ChannelHandlers` that are used for connection creation:
- `TLSEventsHandler` got its own file and unit tests
- `HTTP1ProxyConnectHandler` got its own file and unit tests
- `SOCKSEventsHandler` got its own file and unit tests
- Some small things are already part of this pr that will get their context later. For example:
- `HTTPConnectionPool` is added as a namespace to not cause major renames in follow up PRs
- `HTTPConnectionPool.Connection.ID` and its generator were added now. (This will be used later to identify a connection during its lifetime)
- the file `HTTPConnectionPool+Manager` was added to give `HTTPConnectionPool.Connection.ID.Generator` already its final destination.
Rick Newton-Rogers
David Nadoba
Fabian Fett