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:
At the moment, AHC assumes that creating a `NIOSSLContext` is both cheap
and doesn't block.
Neither of these two assumptions are true.
To create a `NIOSSLContext`, BoringSSL will have to read a lot of
certificates in the trust store (on disk) which require a lot of ASN1
parsing and much much more.
On my Ubuntu test machine, creating one `NIOSSLContext` is about 27,000
allocations!!! To make it worse, AHC allocates a fresh `NIOSSLContext`
for _every single connection_, whether HTTP or HTTPS. Yes, correct.
Modification:
- Cache NIOSSLContexts per TLSConfiguration in a LRU cache
- Don't get an NIOSSLContext for HTTP (plain text) connections
Result:
New connections should be _much_ faster in general assuming that you're
not using a different TLSConfiguration for every connection.
Rick Newton-Rogers
Johannes Weiss