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:
- The properties that store the request body length and the cumulative number of bytes sent as part of a request are of type `Int`.
- On 32-bit devices, when sending requests larger than `Int32.max`, these properties overflow and cause a crash.
- To solve this problem, the properties should use the explicit `Int64` type.
### Modifications:
- Changed the type of the `known` field of the `RequestBodyLength` enum to `Int64`.
- Changed the type of `expectedBodyLength` and `sentBodyBytes` in `HTTPRequestStateMachine` to `Int64?` and `Int64` respectively.
- Deprecated the `public var length: Int?` property of `HTTPClient.Body` and backed it with a new property: `contentLength: Int64?`
- Added a new initializer and "overloaded" the `stream` function in `HTTPClient.Body` to work with the new `contentLength` property.
- **Note:** The newly added `stream` function has different parameter names (`length` -> `contentLength` and `stream` -> `bodyStream`) to avoid ambiguity problems.
- Added a test case that streams a 3GB request -- verified this fails with the types of the properties set explicitly to `Int32`.
### Result:
- 32-bit devices can send requests larger than 2GB without integer overflow issues.
Sometimes it can be useful to connect to one host e.g. `x.example.com` but request and validate the certificate chain as if we would connect to `y.example.com`. This is what this PR adds support for by adding a `dnsOverride` configuration to `HTTPClient.Configuration`. This is similar to curls `—resolve-to` option but only allows overriding host and not ports for now.
### Motivation
If we follow a redirect which changes the origin e.g. from `127.0.0.1` to `localhost` we didn't change the `Host` header to the appropriate new origin and port combination.
### Changes
Use the original request which does not include the host instead of the prepared request to form a new request to the redirect URL.
### Alternatives
If the user defines a `Host` header themselves on the original `HTTPClientRequest` we currently never touch it, even in the redirect case. Maybe we should change our strategy and do one of the following:
1. We could always override the user defined `Host` header
2. We could only remove the user defined `Host` header on redirect and set it to the new origin and port combination
### Motivation
With Xcode 13.2, and therefore Swift 5.5.2, Swift Concurrecy is supported on older Apple OSs. async/await suport will no longer be available on Swift before `5.5.2` but this isn't a breaking change because we have not yet made anything of it public.
### Changes
- replace all `#if compiler(>=5.5) && canImport(_Concurrency)` with `#if compiler(>=5.5.2) && canImport(_Concurrency)`
- replace all `available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)` with `available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)`
Rick Newton-Rogers
aryan-25
Johannes Weiss
David Nadoba