Support Non-Default TrustRoots on Darwin

[PeterAdams-A]

Currently lots of the TLS options are not supported on Darwin - see

async-http-client/Sources/AsyncHTTPClient/NIOTransportServices/TLSConfiguration.swift

Line 115 in c9a9bf0

preconditionFailure("TLSConfiguration.trustRoots != .default is not supported")

Rough instructions of how to implement (thanks @Lukasa)

1. Set a verify block using https://developer.apple.com/documentation/security/2976289-sec_protocol_options_set_verify_.
2. Create a SecTrust from the data you get there: https://github.com/apple/swift-nio-ssl/blob/7932e709af56d27035084ca918bdd729b4084741/Sources/NIOSSL/SecurityFrameworkCertificateVerification.swift#L47
3. Call SecTrustSetAnchorCertificates with the custom roots: https://developer.apple.com/documentation/security/1396098-sectrustsetanchorcertificates
4. Call SecTrustSetAnchorCertificatesOnly with anchorCertificatesOnly: true https://developer.apple.com/documentation/security/1399071-sectrustsetanchorcertificatesonl